A transit is the passage of an astronomical body between an observer and another astronomical body, blocking the view of the further body. The term is more often used if the further body is substantially larger: the term eclipse means about the same thing but is more often used if the nearer body that is blocking the view is nearly the same size or larger. Planets transiting in front of stars are termed transiting planets, such as Venus transiting across the Sun, any solar system planet crossing in front of a star, or an extra-solar planet crossing in front of its own star. Transits (and eclipses) offer opportunities for useful measurement and study: transits of astronomical objects behind the Moon can be used to refine position information and transits of moons in front of Jupiter and Saturn have yielded useful data about the moons and the planets. The transit (e.g., by an exoplanet) is sometimes referred to as the primary eclipse, if the secondary eclipse is also under discussion.
The transit method is one method of detecting exoplanets and binary stars: detecting the magnitude variation of stars due to periodic transits. Transits are also useful for follow-up studies of exoplanets discovered by other means, particularly to determine its radius relative to its host star, to help estimate the planet's density.
The word transit is also used for an astronomical body crossing the field of view of an instrument such as a telescope. In the case of a transit telescope or a meridian circle, it would be a meridian transit, the astronomical body crossing the celestial meridian.