The philtrum (Latin philtrum, Greek, ‘love potion’ ), is a medial cleft common to many mammals, extending from the nose to the upper lip, and, together with a glandular rhinarium (wet nose) and slit-like nostrils, is believed to constitute the primitive condition for mammals in general.
In humans, the philtrum is formed during embryonic development. When these fail to fully develop 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. (Wiki)