Douglas Harper's Etymology Dictionary
dimple in the middle of the upper lip, 1703, medical Latin, from Greek philtron, literally "love charm" (see philtre).
Wiktionary
n. The shallow groove running down the center of the outer surface of the upper lip.
Wikipedia
The philtrum (, philtron), or medial cleft, is a vertical groove in the middle area of the upper lip, common to many mammals, extending in humans from the nasal septum to the procheilon. Together with a glandular rhinarium and slit-like nostrils, it is believed to constitute the primitive condition for mammals in general.
Usage examples of "philtrum".
In the center of his philtrum, the toboggan run in the center of his lip, there was a mole.
If any section of my philtrum is damaged by your blade, the repercussions will be swift and painful.
It was wide and mobile with firm lips, the upper bowing into a sensually pronounced philtrum above the full, sculpted band of the lower.
Her tongue comes out and with no Fred around to watch her (she knows he's watching, she is beset but not stupid ), it does not just pet at her philtrum but slathers it in a great big wipe, like a dog licking its chops after a bowl of scraps.