In linguistics, a morpheme is the smallest semantically meaningful unit in a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word, by definition, is a freestanding unit of meaning. Every word comprises one or more morphemes.

Every morpheme can be classified as either free or bound. These categories are mutually exclusive, and as such, a given morpheme will belong to exactly one of them.

Bound morphemes can be further classified as derivational or inflectional.

Allomorphs are variants of a morpheme that differ in pronunciation but are semantically identical. For example, in English, the plural marker -(e)s of regular nouns can be pronounced /-z/, /-s/, or /-ɨz/, depending on the final sound of the noun's singular form.

In natural language processing for Japanese, Chinese and other languages, morphological analysis is the process of segmenting a sentence into a row of morphemes. Morphological analysis is closely related to part-of-speech tagging, but word segmentation is required for these languages because word boundaries are not indicated by blank spaces.^{[citation needed]}