Conversion

In morphology, conversion is a process which derives words without changing the phonological shape of the base.

Examples
In English it is possible to derive verbs from adjectives in such a way that the verbs are interpreted as 'TO RENDER SOMETHING ADJECTIVE': solid-ify 'make solid', legal-ize 'make legal'. But we also find verbs of this (semantic) type which are phonologically identical to the base adjective: to clean 'make clean', to warm 'make warm' etc. These are instances of conversion.

Comments
Some linguists (e.g. Bloomfield 1933, Kiparsky 1982) assume that converted forms are derived by means of an affix without phonetic content, a so-called zero-affix or null morpheme. Others have challenged this view (e.g. Lieber 1980, 1981).

Synonyms
Other terms for this process are: null affixation, zero-derivation, hypostasis, functional shift, implicit transposition.

Link
Utrecht Lexicon of Linguistics