A morpheme is the smallest meaningful constituent of a linguistic expression. The field of linguistic study dedicated to morphemes is called morphology.

In English, morphemes are often but not necessarily words. Morphemes that stand alone are considered roots (such as the morpheme cat); other morphemes, called affixes, are found only in combination with other morphemes. For example, the -s in cats indicates the concept of plurality but is always bound to another concept to indicate a specific kind of plurality.

This distinction is not universal and does not apply to, for example, Latin, in which many roots cannot stand alone. For instance, the Latin root reg- (‘king’) must always be suffixed with a case marker: rex (reg-s), reg-is, reg-i, etc. For a language like Latin, a root can be defined as the main lexical morpheme of a word.

These sample English words have the following morphological analyses:

• "Unbreakable" is composed of three morphemes: un- (a bound morpheme signifying "not"), break (the root, a free morpheme), and -able (a bound morpheme signifying "an ability to be done").
• The plural morpheme for regular nouns (-s) has three allomorphs: it is pronounced /s/ (e.g., in cats ), /ɪz, əz/ (e.g., in dishes ), and /z/ (e.g., in dogs ), depending on the pronunciation of the root.