concave
https://gyazo.com/4f1e324e9877bc1373af635e66f217dc
source: By DrBob, CC BY-SA 3.0
adjective
having an outline or surface that curves inward like the interior of a circle or sphere:
凹（おう）状の, 凹面の, へこんだ(↔ convex)
e.g. concave lenses. Compare with convex.
DERIVATIVES
concavely adverb
ORIGIN
late Middle English: from Latin concavus, from con- ‘together’ + cavus ‘hollow’.