In linguistics, word formation is the creation of a new word. Word formation is sometimes contrasted with semantic change, which is a change in a single word's meaning. The boundary between word formation and semantic change can be difficult to define: a new use of an old word can be seen as a new word derived from an old one and identical to it in form. See Conversion (word formation). Word formation can also be contrasted with the formation of idiomatic expressions, although words can be formed from multi-word phrases. See Compound (linguistics) and Incorporation (linguistics).