An eclipse occurs when an astronomical object is temporarily blocked by an obstacle, either when another object comes in front of it, or another object comes between the observer and the object. Eclipse is a type of "syzygy."

The term eclipse is often used to describe either a solar eclipse when the Moon's shadow falls on the Earth's surface or a lunar eclipse when the Moon is in the Earth's shadow. However, this term is also used to describe other events that take place beyond the Earth-Moon system. For example, a planet is shaded by its own Moon, and a moon is shaded by its own planet, or another moon shades a moon. A binary system may also form if the observer's position coincides with the level of the star's orbit. Umbra, penumbra, and antumbra are formed when an opaque object obscures a larger light source.