In linguistics, conversion, also called zero derivation, is a kind of word formation; specifically, it is the creation of a word from an existing word without any change in form. Conversion is more productive in some languages than in others; in English it is a fairly productive process.

Often a word of one lexical category (part of speech) is converted from a word of another lexical category; for example, the noun green in golf (referring to a putting-green) is derived ultimately from the adjective green. Conversions from adjectives to nouns and vice versa are both very common and unnotable in English; much more remarked upon is verbing, the creation of a verb by a converting a noun or other word.

The boundary between conversion and functional shift (the extension of an existing word to take on a new syntactic function) is not well-defined.