A morpheme is the smallest meaning-bearing unit of language. The term thus refers to the smallest component of a word that (a) seems to contribute some sort of meaning, or a grammatical function to the word to which it belongs, and (b) cannot itself be decomposed into smaller morphemes.
Kangaroo is one morpheme. Kangaroos is two morphemes, kangaroo and plural -s. The -s expresses the meaning 'many' or 'more than one' in this example.
Morpheme may also be used for 'grammatical morpheme', see morpheme (i.e. grammatical morpheme).
- moneme (Martinet)
The term morpheme was coined by Jan Baudouin de Courtenay in c. 1880. It is based on Greek morph- 'form' and the suffix -eme, on the analogy of the term phoneme. See Mugdan (1986) for detailed discussion.
The concept "morpheme" is not uncontroversial. A number of linguists dispute the explanatory power of the morpheme as a theoretical construct, and also dispute the notion that the morpheme has any psychological reality (cf. Bochner). Consider, for example, the following singular~plural pairs in English: "kangaroo"~"kangaroos"; "mouse"~"mice"; "child"~"children"; "sheep"~"sheep"; "person"~"people".
Mugdan, Joachim. 1986. Was ist eigentlich ein Morphem? In: Zeitschrift für Phonetik, Sprachwissenschaft und Kommunikationsforschung 39(1): 29–43.