conjunction

In astronomy, alignment of two celestial bodies as seen from Earth. A superior planet (or other object) is in conjunction when it lies behind the Sun. An inferior planet (or other object) comes to inferior conjunction when it passes between the Earth and the Sun; it is at superior conjunction when it passes behind the Sun.

Planetary conjunction takes place when a planet is closely aligned with another celestial object, such as the Moon, a star, or another planet.

Because the orbital planes of the inferior planets are tilted with respect to that of the Earth, they usually pass either above or below the Sun at inferior conjunction. If they line up exactly, a transit will occur.