Cupid's bow

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An archery bow that curves inward at the center and usually outward at the ends.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a curved bow with reversed curve ends
• noun the double curve of the upper lip when considered to resemble Cupid's bow