The quality or state of being gibbous; the condition of having a hump or bulge.
From gibbous + -ness (suffix forming abstract nouns from adjectives). This provides an alternative form to 'gibbosity,' both meaning the same thing but with different etymological paths (-ness being Germanic and -osity being Latinate).
English has two ways to say the same thing: 'gibbousness' (from Germanic roots) and 'gibbosity' (from Latin)—this happens constantly in English, giving us fancy and plain words for identical concepts!
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