The coincidence in right ascension of two celestial bodies (one of which is often the Sun). For example, when a planet is at the same right ascension as the Sun, it is said to be in conjunction. Note that the projected positions of the bodies (e.g., sun and planet) need not actually lie on top of each other (they do not necessarily occult) since they may lie at different declinations. The term conjunction is also used more generally to mean a configuration in which two bodies have their least separation.

There is only one type of conjunction for planets outside the Earth's orbit, namely the Earth and planet on opposite sides of the Sun. There are two types of conjunction for planets inside the Earth's orbit. inferior conjunction occurs when the Earth and planet are on the same side of the Sun, and superior conjunction occurs when they are on opposite sides.