The Collaborative International Dictionary of English v.0.48:

Comedo \Com"e*do\, n.; pl. Comedones. [L., a glutton. See
   Comestible.] (Med.)
   A small nodule or cystic tumor, common on the nose, etc.,
   which on pressure allows the escape of a yellow wormlike mass
   of retained oily secretion, with a black head (dirt).
   [1913 Webster]

WordNet (r) 3.0 (2006):

comedo
    n 1: a black-tipped plug clogging a pore of the skin [syn:
         blackhead, comedo]