The term baseline has a particular meaning in interferometry, common in radio astronomy: the distance between two telescopes (e.g., dishes) that comprise part of interferometer. This distance contributes to the angular resolution that can be measured, e.g., through aperture synthesis. For an array of multiple dishes, often a maximum baseline is cited, i.e., the distance between the furthest two dishes, and if they are movable, e.g., on tracks, the distance when they are moved as far as the tracks allow. The potential angular resolution is inversely proportional to the baseline length, but also depends upon the wavelength. This resolution is only for angles of the sky across the direction of the baseline, so multiple dishes with more than one baseline are used, as well as moving dishes around on tracks, and using the rotation of Earth to produce baselines over different directions in relation to the target object.
Baseline is also used in astronomy in a more conventional sense, as a reference level of some measurement, signal or statistic, usually a selected typical level. The term is also used, analogously to the interferometry meaning, for other cases where some distance or measure is of interest, e.g., the distance between two observations in parallax determination, or the time interval between two observations aimed at determining proper motion.