The Collaborative International Dictionary of English v.0.48:

morpheme \mor"pheme\ (m[^o]r"f[=e]m), n. (Linguistics)
   The smallest unit of meaning of a language, which cannot be
   divided into smaller parts carrying meaning; it is usually
   smaller than a single wordform, such as the -ed morpheme of
   verbs in the past tense or the -s morpheme of nouns in the
   plural form.
   [PJC]

WordNet (r) 3.0 (2006):

morpheme
    n 1: minimal meaningful language unit; it cannot be divided into
         smaller meaningful units