In linguistics, a morpheme is the smallest meaningful unit of language.
A morpheme may be an entire word or only part of one. The word “cat” contains one free morpheme because it can stand alone. “Cats” contains two: “cat,” which supplies the lexical meaning, and “-s,” a bound morpheme marking plurality. Likewise, “unhelpful” can be analyzed as “un-,” “help,” and “-ful.”
Morphemes are studied in morphology, the branch of linguistics concerned with word structure. Free morphemes can function independently, while bound morphemes must attach to another form. Affixes such as prefixes and suffixes are common bound morphemes, although some roots are bound as well; the “ceive” in “receive” is not normally used as an independent English word.
A morpheme is not the same as a phoneme, which is the smallest contrastive sound unit and has no meaning by itself. It is also different from a grapheme, a written symbol or letter sequence. Linguists identify morphemes by comparing forms and functions, including grammatical meanings such as tense, number, or comparison.