The Collaborative International Dictionary of English v.0.48:

Taxon \Tax"on\ (t[a^]ks"[o^]n), n.; pl. taxa or taxons.
   a taxonomic group, or the name of a taxonomic grouping.
   [PJC]

WordNet (r) 3.0 (2006):

taxon
    n 1: animal or plant group having natural relations [syn:
         taxonomic group, taxonomic category, taxon]