A Transit is an astronomical body crossing between an observer and an astronomical body. Examples are 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 are useful for a number of types of measurements and studies. Transits of Moons in front of Jupiter and Saturn have yielded useful data.
Exoplanet Eclipse Light Curve
Extra Solar Planet
GJ 1214 b
Trans-Atlantic Exoplanet Survey (TrES)
Transit Timing Variations (TTV)