size of the illuminated portion of a planet or moon.

the ratio of the illuminated area of the apparent disk of a celestial body to the area of the entire apparent disk taken as a circle. For the Moon; phase designations (see lunar phases) are defined by specific configurations of the Sun, Earth and Moon. For eclipses, phase designations (total, partial, penumbral, etc.) provide general descriptions of the phenomena. (See eclipse, solar; eclipse, annular; eclipse, lunar.)

(astronomy) the particular appearance of a body's state of illumination (especially one of the recurring shapes of the part of Earth's moon that is illuminated by the sun); "the full phase of the moon"

The percentage illumination, from the observer's perspective, of an object (normally planet or Moon).

the shape the Moon appears to be as seen from Earth.

The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.

how a planet or moon looks to us at some part of its orbit, when it is lit by the Sun.

The phase of a moon or planet is the proportion of the sunlit side that is visible to an observer. From the Earth, the Moon, Mercury and Venus go through a complete range of phases from 0% of the surface visible to 100% visible. The outer planets show only the gibbous phase. The phase of an astronomical object depends upon the angle between the observer, the Sun and the object being observed.