upper lip; philtrum; under the nose
Refers to the area between the nose and the upper lip, including the philtrum. Often used in expressions like 鼻の下が長い (to be soft on women).
He was sweating on his upper lip.
I traced my philtrum with my finger.
Rarely used alone to mean 'mouth'; more common in set phrases like 鼻の下を伸ばす (to grin foolishly, especially at a woman).
He grinned foolishly when he saw the beautiful woman.