(app:c)=
# Glossary
The terms below are organized by working definition, common boundary cases, and the main places where they are used. Each entry keeps only the distinctions needed for quick reference; the full physical picture, formulas, scales, and references are in the corresponding chapters.

(appsec:c-data)=
## Data, Statistics, and Inference
| Term | Usage in this book | Main locations |
|:--------------|:-------------------------------|:------------------|
| Event table | A photon-by-photon record containing at least time and detection channel, and often frequency, polarization, position, quality flag, and weight. It is not the same object as a binned light curve. | Chapters {ref}`chap:01`, {ref}`chap:06`, and {ref}`chap:07`. |
| Light curve | A projection of the event table onto the time axis, usually discarding channel, polarization, and inter-event information. | Chapters {ref}`chap:02` and {ref}`chap:07`. |
| Exposure function | A function describing how effective observing time, area, efficiency, and selection vary with time or channel. | Chapters {ref}`chap:07` and {ref}`chap:18`. |
| Live time | Effective integration time after removing dead time, bad weather, saturation, and quality cuts. | Chapters {ref}`chap:06` and {ref}`chap:18`. |
| Poisson process | A counting model in which events arrive independently once the instantaneous rate is specified; non-stationary sources require a time-dependent rate. | Chapters {ref}`chap:01`, {ref}`chap:07`, and {ref}`chap:21`. |
| Cox process | A Poisson process whose rate is itself stochastic; it can produce super-Poisson counts without invoking new physics. | Chapter {ref}`chap:21`. |
| Likelihood | The probability density or probability mass for the observed data at fixed model parameters. It should not be confused with the posterior. | Chapters {ref}`chap:07` and {ref}`chap:18`. |
| Posterior | The parameter distribution obtained by combining likelihood and prior. Reports of posteriors should state both the prior and the data vector. | Chapters {ref}`chap:07` and {ref}`chap:20`. |
| Fisher information | A measure of the local sensitivity of the likelihood to a parameter; it is not an automatic substitute for a systematic-error model. | Chapters {ref}`chap:01`, {ref}`chap:07`, and {ref}`chap:08`. |
| Covariance matrix | A matrix describing correlations among data points or parameter errors. Multi-baseline $|V|^2$ points should not be assumed independent by default. | Chapters {ref}`chap:07` and {ref}`chap:18`. |
| Fano factor | The ratio of count variance to count mean, useful for diagnosing slow source variability, dead time, and afterpulsing. | Chapter {ref}`chap:21`, Eq. {eq}`eq:ch21-fano`. |
| Trial factor | The global-significance correction caused by multiple testing. Time bins, frequency channels, baselines, and target counts can all contribute. | Chapter {ref}`chap:21`. |
| Null test | A check designed to remove the astrophysical signal while retaining instrumental or background structure, such as a time-shift, off-source, off-band, or crossed-polarization test. | Chapters {ref}`chap:07`, {ref}`chap:20`, and {ref}`chap:21`. |

(appsec:c-optics)=
## Quantum Optics and Coherence Functions
| Term | Usage in this book | Main locations |
|:--------------|:-------------------------------|:------------------|
| Mode | A distinguishable degree of freedom of the optical field, such as a spatial, temporal, frequency, or polarization mode. Mode number controls how strongly bunching contrast is diluted. | Chapters {ref}`chap:03` and {ref}`chap:04`. |
| Occupation number | Mean photon number in a single mode; it is often large in radio astronomy and very small for one optical astronomical mode. | Chapters {ref}`chap:03` and {ref}`chap:09`. |
| Coherent state | An ideal optical state with Poisson photon-number distribution and $g^{(2)}(0)=1$. Astrophysical laser or maser candidates are not automatically stable coherent states. | Chapters {ref}`chap:03` and {ref}`chap:21`. |
| Thermal state | A chaotic optical field with bunching in a single mode; multimode averaging and finite response lower the observed contrast. | Chapters {ref}`chap:03` and {ref}`chap:04`. |
| Squeezed state | A field state in which one quadrature has noise below the vacuum level and the conjugate quadrature has increased noise; related language also appears in cosmological perturbations. | Chapters {ref}`chap:03` and {ref}`chap:16`. |
| Density matrix | The state object for pure or mixed states; especially natural when astronomical light is a mixture of many unresolved components. | Chapters {ref}`chap:03` and {ref}`chap:17`. |
| Decoherence | Loss of usable phase relations through environment, averaging, or unobserved degrees of freedom; it should not be reduced to "not quantum." | Chapters {ref}`chap:03` and {ref}`chap:16`. |
| First-order coherence function | The field-amplitude correlation; amplitude interferometry and complex visibility depend on it. | Chapters {ref}`chap:04` and {ref}`chap:05`. |
| Second-order coherence function | The intensity or detection-event correlation; the central observable for HBT and photon statistics. | Chapters {ref}`chap:04` and {ref}`chap:07`. |
| Bunching | A positive second-order correlation excess near zero delay, characteristic of thermal or chaotic light. | Chapters {ref}`chap:01` and {ref}`chap:04`. |
| Antibunching | A nonclassical photon-statistics signature with $g^{(2)}(0)<1$; very difficult to preserve in astronomical environments. | Chapters {ref}`chap:04` and {ref}`chap:21`. |
| Coherence time | The temporal width over which first-order coherence is appreciable, usually set by spectral bandwidth. | Chapters {ref}`chap:01` and {ref}`chap:04`. |
| Siegert relation | The relation, valid for thermal light or Gaussian fields, connecting second-order correlation to the squared modulus of first-order coherence. | Chapters {ref}`chap:01`, {ref}`chap:04`, and {ref}`chap:05`. |
| Glauber correlation | A quantum-optical correlation function defined with detection operators; it underlies the $g^{(n)}$ notation. | Chapters {ref}`chap:03` and {ref}`chap:04`. |

(appsec:c-interferometry)=
## Interferometry, Imaging, and Mode Measurements
| Term | Usage in this book | Main locations |
|:--------------|:-------------------------------|:------------------|
| Complex visibility | The normalized Fourier component of the sky brightness distribution; both amplitude and phase are first-order coherence information. | Chapters {ref}`chap:01` and {ref}`chap:05`. |
| Baseline | The vector separation between two telescopes; the visibility depends on the sky-projected component $B_\perp$. | Chapter {ref}`chap:05`. |
| $u,v$ coverage | Sampling of the spatial-frequency plane; it controls imaging fidelity and model degeneracy. | Chapters {ref}`chap:05` and {ref}`chap:18`. |
| VCZ theorem | The theorem connecting the sky brightness of an incoherent far-field source to first-order spatial coherence. | Chapter {ref}`chap:05` and Appendix {ref}`app:d`. |
| Uniform disk | The simplest model for a stellar angular diameter; real stars often require limb darkening or departures from spherical symmetry. | Chapters {ref}`chap:01`, {ref}`chap:05`, and {ref}`chap:10`. |
| Limb darkening | The decrease of stellar-disk brightness toward the limb, which changes the visibility curve and the interpretation of angular diameter. | Chapter {ref}`chap:10`. |
| Intensity interferometry | Measurement of $|V|^2$ through correlations of intensity fluctuations. It is insensitive to atmospheric piston phase, but sensitive to background, timing response, and calibration. | Chapters {ref}`chap:05` and {ref}`chap:18`. |
| Zero-baseline contrast | The correlation-peak amplitude before spatial resolution suppresses it, often used to calibrate instrumental dilution. | Chapters {ref}`chap:05` and {ref}`chap:18`. |
| Phase loss | The loss of direct complex phase information when only $|V|^2$ is measured, leading to mirror and asymmetry degeneracies. | Chapters {ref}`chap:05` and {ref}`chap:21`. |
| Phase retrieval | The algorithmic problem of recovering phase from intensity or $|V|^2$ constraints, requiring priors and multiple samples. | Chapters {ref}`chap:05` and {ref}`chap:21`. |
| SPADE | Spatial-mode demultiplexing: a measurement idea that projects a focal-plane field onto spatial modes to estimate sub-Rayleigh separations. | Chapters {ref}`chap:08` and {ref}`chap:20`. |
| Rayleigh curse | The drop in Fisher information for estimating the small separation of two sources with direct imaging; it is not a limit on all measurements. | Chapters {ref}`chap:08` and {ref}`chap:21`. |
| Quantum Fisher information | The upper bound on parameter information over an allowed measurement set; it is reachable only when the model and measurement assumptions are satisfied. | Chapter {ref}`chap:08`. |

(appsec:c-instrument)=
## Instruments, Detectors, and Calibration
| Term | Usage in this book | Main locations |
|:--------------|:-------------------------------|:------------------|
| SPAD | Single-photon avalanche diode, useful for time-tagged photon counting; dead time and afterpulsing must be modeled. | Chapter {ref}`chap:06`. |
| PMT | Photomultiplier tube, often used for fast photon counting and Cherenkov instruments; gain, pulse shape, and background require calibration. | Chapters {ref}`chap:06` and {ref}`chap:18`. |
| TDC | Time-to-digital converter, which turns detector pulses into digital time tags. | Chapters {ref}`chap:06` and {ref}`chap:20`. |
| Jitter | Timing scatter introduced by the detection and timing chain, broadening correlation peaks and diluting contrast. | Chapter {ref}`chap:06`. |
| Dead time | The interval after one event during which a detector or electronic chain cannot record a new event. | Chapters {ref}`chap:06` and {ref}`chap:21`. |
| Afterpulsing | Delayed spurious events caused by trap release inside a detector, producing positive short-delay correlations. | Chapters {ref}`chap:06` and {ref}`chap:21`. |
| Dark counts | Detection events in the absence of astrophysical photons; temperature and bias voltage can change them. | Chapter {ref}`chap:06`. |
| Electronic crosstalk | Contamination of one channel by the electronic pulse of another, potentially mimicking a zero-delay correlation. | Chapters {ref}`chap:06` and {ref}`chap:21`. |
| White Rabbit | A network-synchronization technology capable of sub-ns timing; adequacy depends on correlation-peak width and the baseline model. | Chapter {ref}`chap:06`. |
| Spectral channel | A frequency or wavelength label in the event table; narrower channels increase coherence time but reduce per-channel photon number. | Chapters {ref}`chap:06` and {ref}`chap:22`. |
| Polarization channel | A polarization label in the event table, used to separate source physics, scattering, Faraday effects, and instrumental polarization. | Chapters {ref}`chap:06` and {ref}`chap:15`. |
| Calibrator star | A reference star used to determine zero-baseline contrast, system response, or angular-diameter scale. | Chapters {ref}`chap:18` and {ref}`chap:22`. |
| Systematic floor | A lower error bound that no longer improves as $T^{-1/2}$ when integration time is increased. | Chapters {ref}`chap:18` and {ref}`chap:21`. |

(appsec:c-sources)=
## Astrophysical Sources and Radiation Mechanisms
| Term | Usage in this book | Main locations |
|:--------------|:-------------------------------|:------------------|
| Brightness temperature | Radiation intensity expressed as an equivalent blackbody temperature; very high brightness temperature often points to coherent emission or an extremely small angular scale. | Chapter {ref}`chap:09`. |
| Thermal radiation | Radiation controlled by temperature and optical depth; optical stellar light is approximately thermal but not a perfect blackbody. | Chapters {ref}`chap:09` and {ref}`chap:10`. |
| Synchrotron radiation | Radiation from relativistic electrons in magnetic fields, usually polarized and nonthermal. | Chapters {ref}`chap:09` and {ref}`chap:12`. |
| Inverse Compton scattering | The process by which energetic electrons boost low-energy photons to higher energies. | Chapter {ref}`chap:09`. |
| Maser | Microwave or radio stimulated-emission source, often controlled in astrophysical environments by velocity coherence and pump fluctuations. | Chapters {ref}`chap:09` and {ref}`chap:21`. |
| Natural laser | A possible optical or near-infrared stimulated-emission candidate in an astrophysical environment; not the same as a stable laboratory laser cavity. | Chapters {ref}`chap:19` and {ref}`chap:21`. |
| Broad-line region | The gas region producing broad emission lines in an AGN, constrained jointly by time delay, spectra, and angular scale. | Chapter {ref}`chap:12`. |
| Photon ring | Fine structure near a black hole produced by paths associated with photon orbits in strong gravity. | Chapter {ref}`chap:12`. |
| Type Ia supernova | A thermonuclear supernova used as a distance indicator; the intensity-interferometry scheme focuses on the photospheric angular radius. | Chapters {ref}`chap:13` and {ref}`chap:20`. |
| Kilonova | An r-process transient following a binary-neutron-star merger; multi-messenger delay and diffusion time are key quantities. | Chapter {ref}`chap:13`. |
| GRB afterglow | Multiwavelength emission produced as a gamma-ray-burst jet interacts with its environment. | Chapter {ref}`chap:13`. |
| TDE | A tidal-disruption event, in which a star is torn apart by a black hole; it can constrain black-hole mass and the reprocessing photosphere. | Chapter {ref}`chap:13`. |

(appsec:c-propagation)=
## Propagation, Cosmology, and New Physics
| Term | Usage in this book | Main locations |
|:--------------|:-------------------------------|:------------------|
| DM | Dispersion measure, the frequency-dependent delay caused by electron column density. | Chapter {ref}`chap:14`. |
| RM | Rotation measure, the Faraday polarization rotation produced by magnetized plasma. | Chapters {ref}`chap:14` and {ref}`chap:15`. |
| Scattering broadening | Multipath propagation that broadens a pulse or correlation peak and can hide intrinsic time structure. | Chapter {ref}`chap:14`. |
| Extinction | Flux reduction by dust absorption and scattering; it affects photon rate and color. | Chapter {ref}`chap:14`. |
| Lens equation | The relation among source position, image position, and deflection angle. | Chapter {ref}`chap:14`. |
| Time-delay distance | The distance combination entering time-delay cosmology with strong lenses. | Chapter {ref}`chap:14`. |
| Wave-optics lensing | Lensing in the regime where wavelength, lens mass, and path difference make interference non-negligible. | Chapters {ref}`chap:14` and {ref}`chap:15`. |
| Axion birefringence | A candidate effect in which a light field coupling changes polarization angle with time or direction. | Chapters {ref}`chap:15` and {ref}`chap:16`. |
| CMB B modes | The curl-like component of CMB polarization, produced by gravitational waves, lensing, or systematics. | Chapter {ref}`chap:16`. |
| Cosmic variance | The large-scale statistical uncertainty caused by observing only one universe. | Chapter {ref}`chap:16`. |
| Standard siren | A source whose gravitational-wave signal gives distance directly, then combines with redshift or an electromagnetic counterpart for cosmology. | Chapter {ref}`chap:13`. |

(appsec:c-program)=
## Quantum Networks and Program Management
| Term | Usage in this book | Main locations |
|:--------------|:-------------------------------|:------------------|
| Quantum-network telescope | A future architecture in which entanglement, quantum memories, frequency conversion, or local mode measurements assist long-baseline interferometry. | Chapters {ref}`chap:17` and {ref}`chap:22`. |
| Entanglement distribution rate | The usable number of entangled resources supplied by a network per second; link loss suppresses it rapidly. | Chapter {ref}`chap:17`. |
| Quantum memory | A device that stores quantum states or entanglement resources; storage time must cover baseline light-travel time and processing delay. | Chapter {ref}`chap:17`. |
| Bell fidelity | The fidelity between the actual entangled state and an ideal Bell state, used as a measure of network-resource quality. | Chapter {ref}`chap:17`. |
| Frequency conversion | Conversion of astronomical light or laboratory quantum resources into bands that can be transmitted, stored, or detected. | Chapter {ref}`chap:17`. |
| Readiness | A quantitative judgment of whether the observable, instrument metric, calibration, code, and data products are mature enough for execution. | Chapter {ref}`chap:22`. |
| Milestone | A project step with an observable, target precision, deadline, null test, and data deliverable. | Chapter {ref}`chap:22`. |
| Data product | A reusable deliverable such as an event table, correlation histogram, $|V|^2$, covariance, or posterior samples. | Chapter {ref}`chap:22`. |
| Risk register | A project table listing technical risks, science risks, probabilities, impacts, and mitigation plans. | Chapter {ref}`chap:22`. |
| Failure criterion | A pre-defined condition for stopping or downgrading a project, such as a target being too faint, a systematic floor being too high, or a trigger arriving too late. | Chapters {ref}`chap:18` and {ref}`chap:22`. |