A transit occurs when one celestial body passes across the face of another, as when the inferior planets (those whose orbits lie within that of the Earth‘s, i.e. Mercury or Venus) pass in front of the Sun‘s disc.
Transits of moons across the face of their parent planets, such as Jupiter and Saturn, can also be observed with a good amateur telescope.
Transits of extrasolar planets have been studied, as they pass in front of their parent stars, causing a dimming of the star’s light as viewed from Earth.
Transits between other bodies would also be observable from different points in space.
If the majority or all of the background object is obscured by the foreground object, this is instead referred to as an occultation.
N.B in astronomy the term transit can also refer to the a celestial object crossing a meridian, or the movement of an object across the viewfinder of a telescope.