The Collaborative International Dictionary of English v.0.48:

Epizoon \Ep`i*zo"["o]n\, n.; pl. Epizoa. [NL., fr. Gr. 'epi`
   upon + zw^,on animal.] (Zo["o]l.)
   One of the artificial group of invertebrates of various
   kinds, which live parasitically upon the exterior of other
   animals; an ectozo["o]n. Among them are the lice, ticks, many
   acari, the lerneans, or fish lice, and other crustaceans.
   [1913 Webster]

WordNet (r) 3.0 (2006):

epizoon
    n 1: any external parasitic organism (as fleas) [syn:
         ectoparasite, ectozoan, ectozoon, epizoan,
         epizoon]