Conversion, also known as zero derivation or null derivation in linguistics, is a type of word formation that entails the development of a new word (of a different word class) from an existing word (of a different word class) with no change in form, or derivation using just zero.

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In English grammar, conversion is a word-formation process that assigns an existing word to a different word class, part of speech, or syntactic category. This process is also called zero derivation or a functional shift.