The Collaborative International Dictionary of English v.0.48:

hyponym \hyponym\ n. (linguistics)
   a word that is more specific (less abstract) than a given
   word; a term designating a class which is a subtype of the
   given word; a subcategory; a subtype. Inverse of hypernym;
   as, "dog" is a hyponym of "mammal", and "mammal" is a hyponym
   of "vertebrate".

   Syn: subordinate, subordinate word.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

hyponym
    n 1: a word that is more specific than a given word [syn:
         hyponym, subordinate, subordinate word]