Adjective meaning curved inward, like the inside of a circle or sphere
... concave [entry 1, 
adjective] | hollow [synonym, sense-specific] | dented [synonym, ... having an 
outline or surface that curves inwards like the interior of a circle or sphere (16 of 
61... [DOC] [TLE] Concave - definition of concave by The Free Dictionary [PAR] Define concave.