名詞 concavity:

1. a shape that curves or bends inward
   同義詞：concave shape, incurvation, incurvature
2. the property possessed by a concave shape
   同義詞：concaveness

con.cav.i.ty

n.（名詞）
【復數】 con.cav.i.ties
The state of being curved like the inner surface of a sphere.
凹陷：被刻成象球體內表面的樣子
A surface or structure configured in such a curve:
被制成這樣彎曲的表面或結構：
例句：
.a perfect concavity of white sand lined with palm . . . trees.(Islands)
.一個完美的邊上長著棕櫚樹的白色沙灘.(島嶼)