gibbous

Gibbous is a term used in astronomy to describe a phase of an illuminated celestial body, most commonly the Moon, when the visible portion of its disk is more than half but not yet full. The gibbous phase occurs after the first quarter during the waxing sequence and before the last quarter during the waning sequence. When the Moon is waxing gibbous, the illuminated area increases toward full; when waning gibbous, it decreases from full toward a half-lit shape again.

The term derives from Latin gibbus meaning “hump” or “curved,” passing through Old French and into Middle English. This origin reflects the bulging, rounded appearance of the sunlit portion of the body.

In usage, gibbous describes any body with illumination between 50 and 100 percent. The Moon is the primary example, though in principle other planets or spacecraft can display gibbous-like phases under favorable lighting. The precise orientation of the gibbous shape depends on the observer’s latitude and the time of night, so the bulging edge may appear on different sides to observers in different hemispheres.