concave
adj.curving inward like the inside of a bowl or a spoon. It is the opposite of convex, which curves outward.
adj.curving or hollowing inward; having a surface that is lower in the middle than at the edges. Often used in optics to describe lenses that diverge light.
“The inside of a spoon has a concave shape.”
“The architect designed a concave wall to help focus the sound toward the center of the auditorium.”
“A concave lens causes parallel rays of light to spread out, making it an essential component in corrective eyewear for people with nearsightedness.”
Typically used attributively before a noun or predicatively after a linking verb like 'is' or 'appears'.
Learners frequently confuse 'concave' with 'convex'; remember that 'cave' implies a hollow space going inward.
From Middle English concave, from Old French concave, from Latin concavus.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.