In linguistics, a compound is a lexeme (less precisely, a word) that consists of more than one stem. Compounding, composition or nominal composition is the process of word formation that creates compound lexemes. That is, in familiar terms, compounding occurs when two or more words are joined to make one longer word. The meaning of the compound may be similar to or different from the meanings of its components in isolation. The component stems of a compound may be of the same part of speech—as in the case of the English word footpath, composed of the two nouns foot and path—or they may belong to different parts of speech, as in the case of the English word blackbird, composed of the adjective black and the noun bird. With very few exceptions, English compound words are stressed on their first component stem.

The addition of affix morphemes to words (such as suffixes or prefixes, as in employ → employment) should not be confused with nominal composition, as this is actually morphological derivation.