The Word of the Day comes from "Transcendent Kingdom" by Yaa Gyasi. It is shortlisted for the 2021 Tournament of Books.
PHILTRUM
The philtrum (Latin: philtrum from Ancient Greek φίλτρον phíltron, lit. "love charm"), or medial cleft, is a vertical indentation in the middle area of the upper lip, common to many mammals, extending in humans from the nasal septum to the tubercle of the upper lip.
In most mammals, the philtrum is a narrow groove that may carry dissolved odorants from the rhinarium or nose pad to the vomeronasal organ via ducts inside the mouth.
For humans and most primates, the philtrum survives only as a vestigial medial depression between the nose and upper lip.
The human philtrum, bordered by ridges, also is known as the infranasal depression, but has no apparent function. That may be because most higher primates rely more on vision than on smell. Strepsirrhine primates, such as lemurs, still retain the philtrum and the rhinarium, unlike monkeys and apes.