A morpheme is the smallest meaningful lexical item in a language. A morpheme is not necessarily the same as a word. The main difference between a morpheme and a word is that a morpheme sometimes does not stand alone, but a word, by definition, always stands alone. The field of linguistic study dedicated to morphemes is called morphology.

In English, when a morpheme can stand alone, it is considered a root because it has a meaning of its own (such as the morpheme cat). When it depends on another morpheme to express an idea, it is an affix because it has a grammatical function (such as the –s in cats to indicate plurality).[1]. However this definition is not universal and it does not apply to, e.g. Latin where 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. In a language like Latin, a root can be defined as the main lexical morpheme of a word.

  • "Unbreakable" is composed of three morphemes: un- (a bound morpheme signifying "not"), -break- (the root, a free morpheme), and -able (a free morpheme signifying "can be done").
  • Allomorphs of the plural morpheme for regular nouns: /s/ (e.g., in cats /kæts/), /ɪz, əz/ (e.g., in dishes /dɪʃɪz/), and /z/ (e.g., in dogs /dɒɡz/).