What is the smallest meaningful unit of language called?

The smallest meaningful unit of language is called a morpheme. Morphemes are the basic units studied by morphology, the branch of linguistics concerned with word structure.

Some morphemes can stand alone as words. These are free morphemes, such as “cat,” “book,” or “run.” Others are bound morphemes and must attach to another form. In “cats,” the plural ending “-s” is a bound morpheme because it contributes the meaning “more than one” but cannot normally function as an independent word.

A single word can contain several morphemes. “Unhappiness,” for example, can be analyzed as “un-,” “happy,” and “-ness”: a negative prefix, a root expressing a state, and a suffix forming a noun. A morpheme is not the same as a syllable or phoneme. Syllables are units of pronunciation, while phonemes are contrastive speech sounds; neither necessarily carries meaning.

Morpheme boundaries can be subtle. A morpheme may have different forms in different contexts, such as the plural endings in “cats,” “dogs,” and “horses.” Linguists call such variant forms allomorphs.