In morpheme-based morphology, a morpheme is the smallest linguistic unit that has semantic meaning.

In spoken language, morphemes are composed of phonemes, the smallest linguistically distinctive units of sound.

The concept morpheme differs from the concept word, as many morphemes cannot stand as words on their own. A morpheme is free if it can stand alone, or bound if it is used exclusively alongside a free morpheme.

English example: The word "unbreakable" has three morphemes "un-", (meaning not x) a bound morpheme, "-break-" a free morpheme, and "-able". "un-" is also a prefix, "-able" is a suffix. Both are affixes.