The philtrum (Latin philtrum, Greek φίλτρον philtron, 'love potion' ), is a medial cleft common to many mammals, extending from the nose to the upper lip, and, together with a glandular rhinarium and slit-like nostrils, is believed to constitute the primitive condition for mammals in general.  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, is also known as the infranasal depression, but has no apparent function.
In humans, the philtrum is formed where the nasomedial and maxillary processes meet during embryonic development. When these processes fail to fuse fully in humans, a cleft lip (sometimes called a "hare lip") can result. A flattened or smooth philtrum can be a symptom of Fetal alcohol syndrome or Prader-Willi syndrome.
According to the Jewish Talmud (Niddah 30b), God sends an angel to each womb and teaches a baby "the entire Torah". Before the baby is born, the angel touches it between the upper lip and the nose and all that was learned is forgotten; the philtrum is formed when the angel "shushes" the baby to cause it to forget its holy knowledge. Other stories say that it is an indent left by the finger of God.