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

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "morpheme":
   IC analysis, accidence, affix, affixation, allomorph,
   bound morpheme, conjugation, cutting, declension, derivation,
   difference of form, enclitic, form, formative, free form, glosseme,
   icon, immediate constituent analysis, infix, infixation,
   inflection, lexeme, lexical form, morph, morphemic analysis,
   morphemics, morphology, morphophonemics, paradigm, phrase, prefix,
   prefixation, proclitic, radical, root, semasiological unit, sememe,
   sign, signifiant, significant, stem, suffix, suffixation, symbol,
   term, theme, token, type, word, word-formation