For an object in the solar system, the phase angle is the angle 'Sun-object-Earth' that is, the angle between the Sun and the observer as seen from the given object. It is 0° when the object is fully illuminated, 90° when the object is half-illuminated (like the Moon at first quarter and last quarter), and 180° when the object is between Earth and the Sun (like the Moon at new moon). The Moon, Mercury, and Venus can have phase angles covering the full range 0° to 180°. Mars, on the other hand, has a maximum phase angle of about 45°, meaning that it is always almost fully illuminated.

Phase function is the change in the brightness of an object as a function of the phase angle. In general, an object gets brighter as the phase angle approaches 180° or 0°. The function is usually fairly smooth except for at small phase angles where there may be a 'spike' of increased brightness. The phase function is usually described as the change in magnitude (brightness) per degree of phase angle. Phase defect is the angular extent of the illuminated portion of the disk of the Moon or a planet compared to its full disk.