| Definition -
A word-formation process whereby a new word is created by using an existing word as a different part of speech, e.g. using a noun as a verb.
1. Because English has lost so many of its inflections, this kind of language change happens easily and often.
Example - When it entered English from Norman French in the 11th century, the word market was a noun that solely denoted a place where sales or barter occurred (e.g. "I went to the market"). The word now can also be used as a verb denoting the act itself of selling or promoting (e.g. "I marketed my board game").
Oxford English Dictionary - The word's first citation in this sense is from 1928:
"It seems to me more logical to say that the adjective is converted and to call the result a conversion-noun than to use the term converted noun in the latter sense."
(Bergener, Conversion of Adjectives into Nouns, 2)