In linguistics, conversion, also called zero derivation, is a kind of word formation; specifically, it is the creation of a word (of a new word class) from an existing word (of a different word class) without any change in form. 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 the creation
of a verb by converting a noun or other word (e.g., the adjective clean becomes the verb to clean).