morpheme

A morpheme is the smallest unit of language that carries meaning or grammatical function. It is a theoretical construct used in morphology to describe how words are formed and understood. Morphemes may be free, able to stand as words, or bound, requiring attachment to another form. Examples: "book" is a free morpheme; "books" consists of free morpheme "book" plus bound morpheme "s" marking plural.

A word can contain several morphemes: "unhappiness" = "un-" (bound derivational prefix), "happy" (free lexical morpheme), "-ness" (bound derivational suffix). Inflectional morphemes modify grammatical category but do not change core meaning, e.g., plural -s, past -ed; derivational morphemes create new words or word classes, e.g., "happy" to "happiness" or "teach" to "teacher".

Allomorphy: a single morpheme may have different phonetic realizations; e.g., plural morpheme is realized as [s], [z], or [ɪz] as in "cats", "dogs", "horses". A zero morpheme may occur when a form has no additional affix, as in "sheep" plural.

In analysis, morphemes are the building blocks of stems and affixes; languages vary in morphology, from isolating (few affixes) to agglutinative or fusional systems. Lexical morphemes convey content; grammatical morphemes provide function.