functional shift; conversion
Linguistics term for when a word changes its part of speech without affixation, e.g., a noun used as a verb.
A noun being used as a verb is an example of functional shift.
品詞転換 is a broader term for part-of-speech conversion, while 機能転換 specifically refers to functional shift without morphological change.
Compound of 機能 (function) and 転換 (conversion, shift), likely a calque from English 'functional shift'.