noun: The vertical groove below the nose and above the upper lip.
From Latin philtrum (love potion, groove under the nose), from Greek philtron (love potion, groove under the nose). Earliest documented use: 1609.
“John Ryan Fitzpatrick flexed his index finger under his nose and across his philtrum.”
Jeff Hicks; Troubles at The Glen; Waterloo Region Record (Canada); Aug 7, 2010.