Definition of "philtrum"

Oral intercommissural distance (mouth width) and philtrum length are useful measurements for diagnosis of wide or large mouth (macrostomia), small mouth (microstomia), long philtrum and short philtrum and form ‘good handles’ in syndromic delineation.

2005, M. L. Kulkarni, Physical Examination of Children (Clinical Methods in Paediatrics; 2B)‎, India: Jaypee Brothers, page 142

Upper lip reconstruction is more difficult than lower lip reconstruction because the upper lip has a central structure, the philtrum. […] Wounds adjacent to the philtrum that are closed primarily may shift the philtrum to the affected side. However, with time, the philtrum will tend to return to its midline position.

2007, Loren J. Borud, Arin K. Greene, “Chapter 24: Lip Reconstruction”, in Josef E. Fischer, Kirby I. Bland, Mark P. Callery, editors, Mastery of Surgery‎, 5th edition, volume 1, page 301, column 3

If the philtrum is long and deep, then these people are very romantic, sexual, lively and friendly.

2011, Hrishikesh Dubey, Mysteries of Vedic Face Reading‎, Categorization of the Philtrum According to Its Length