The Moon is in the full moon phase when it appears fully illuminated from Earth.
A full moon occurs when Earth lies approximately between the Sun and the Moon, allowing sunlight to illuminate nearly the entire lunar hemisphere facing us. The Moon does not produce its own visible light; its bright appearance comes from reflected sunlight.
Full moon is one of the four principal lunar phases, alongside new moon, first quarter, and last quarter. It occurs about once every 29.53 days, the average length of the synodic month. The Moon is technically full only at a particular moment, although it can look fully illuminated for a night or more to casual observers.
A gibbous moon is more than half illuminated but not completely full, while a crescent moon is less than half illuminated. A new moon is the opposite phase: the side facing Earth is mostly unlit and the Moon is difficult to see. Lunar eclipses can occur near full moon when the Moon passes through Earth’s shadow.