The crescent moon (a thinly curved moon a few days after new moon); also used metaphorically for crescent-shaped objects or smiles.
今夜は空にきれいな三日月が出ている。
A beautiful crescent moon is visible in the sky tonight.
彼女の口元はいつも三日月のような笑顔だ。
Her smile always looks like a crescent.
三日月形のネックレスを買った。
I bought a crescent-shaped necklace.