Definition of os externum in US English:

os externum

noun

Anatomy
rare
  • Originally: †the external opening of the vagina (obsolete). In later use: the opening of the cervix into the vagina; the os uteri.

Origin

Mid 18th century; earliest use found in William Smellie (1697–1763), man-midwife. From post-classical Latin os externum from classical Latin ōs + externum, neuter of externus outer.