Cupid's bow

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An archery bow that curves inward at the center and usually outward at the ends.

Wiktionary

1. n. The archery bow borne by Cupid in artistic representations, which curves inward at the centre and often outward at the ends.
2. n. The curve of the upper lip which resembles such a bow.
3. n. A similar curve carved in woodwork, etc.

WordNet 3.0

1. n. a curved bow with reversed curve ends
2. n. the double curve of the upper lip when considered to resemble Cupid's bow