The philtrum is an oddly specific anatomical term, meaning “the vertical indentation in the midline of the upper lip, also called the infranasal depression.” I never knew there was even a word for that!
It comes from Greek philein, meaning “to love”. According to Wikipedia, “the ancient Greeks used to believe that the philtrum was one of the most erogenous spots on the human body”. There are stories that say it’s created by the touch of an angel, “shushing” the baby to make it forget the angel’s wisdom. But the more likely explanation is that the philtrum exists to give us a wider range of lip motions for better communication.