epizoon

from WordNet (r) 3.0 (2006)

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

from 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]