What does gibbous mean?
Gibbous is an adjective describing something rounded or bulging outward, especially a shape that is convex rather than flat or concave. Its best-known use is astronomical: a gibbous Moon is in a phase between half moon and full moon, when more than half of the visible surface is illuminated. The illuminated portion retains a curved outline, but the shadow no longer forms the straight dividing line seen at half moon. As the Moon waxes toward full, it appears increasingly round; after full, the gibbous shape recedes as it wanes toward last quarter. Outside discussions of the lunar cycle, gibbous can describe rounded, swollen, or outward-projecting forms, though this use sounds more literary than everyday. The word is useful because it names both a precise phase in the Moon’s cycle and a broader visual quality of bulging curvature.
adjectiveRounded or bulging outward. Of the Moon, convex in shape because more than half of its visible surface is illuminated.
- Rounded, bulging, or protuberant in shape.
- Of the Moon or a lunar phase, appearing convex because more than half of the visible surface is illuminated.
"A gibbous moon cast a soft, silver light over the harbor."
"The gibbous moon rose above the dark hills."
"The ancient bowl has a smooth, gibbous shape."
Between half and full, the Moon becomes gibbous—more than half illuminated, but not yet round.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does gibbous come from?
Gibbous entered English in the early 17th century from Latin gibbōsus, meaning swollen or bulging and based on gibbus, a swelling or prominence. The word belongs to a family of forms expressing an outward curve or protuberance, including the English gib. Its specialized lunar meaning developed from the general sense of a convex, bulging form.
Related word forms
How gibbous is actually used
The astronomical sense is standard and precise. Outside astronomy, the word is relatively uncommon and often has a formal or literary quality.
Easily confused with gibbous
Gibbous means bulging or convex, whereas a gibbon is a type of ape.