A telluric star (or telluric standard star or standard star or just telluric) is a star with a known spectrum which is used for calibrating ground observations.
The goal is to work around telluric contamination, the affects of Earth atmosphere on observation, particularly in spectroscopy, given that Earth atmosphere adds absorption lines. A challenge is that the degree and details of contamination depend strongly upon your location (e.g., your altitude), the angle of view, and weather conditions. The strategy is, in effect, to measure the contamination when/where you observe: when you observe an object (e.g., a star or galaxy), you also observe (at the same time or immediately before or after) a nearby star for which a spectrum is well documented, i.e., your telluric star. Using the difference between its known spectrum and your observation of it, you account for the affect of the atmosphere on your intended observation. The ideal telluric star is angularly close to your object-of-interest, has a well-documented, accurate spectrum, is bright (or at least not too dim), and has few spectral features, i.e., a minimum of spectral lines, continuous absorption regions, etc.