Adjective meaning curved inward, like the inside of a circle or sphere
[DOC] [TLE] concave - Dictionnaire de l'Acadmie franaise - Academic [PAR] + cavus, hollow: see CAVE] hollow and curved like the inside of a bowl n. a 
concave ... concave  means 'having an outline or surface like the interior of a 
circle or sphere', whereas convex means 'having an outline ... concave   
ADJECTIVE  having an outline or surface that curves inwards like the interior of 
a sphere.