concave
US /ˈkɑːn.keɪv/
UK /ˈkɑːn.keɪv/

1.
concaaf, hol
having an outline or surface that curves inward like the interior of a circle or sphere
:
•
The spoon has a concave shape.
De lepel heeft een concave vorm.
•
The lens of the telescope is concave.
De lens van de telescoop is concaaf.