epizoon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An epizoic organism.

from The Century Dictionary.

• noun One of the Epizoa; an epizoan.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) One of the artificial group of invertebrates of various kinds, which live parasitically upon the exterior of other animals; an ectozoön. Among them are the lice, ticks, many acari, the lerneans, or fish lice, and other crustaceans.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology An external animal parasite

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any external parasitic organism (as fleas)