Null affixation

Null affixation is a process by which new words are formed by adding an affix which happens to be phonologically null. Many linguists (e.g. Bloomfield (1933), Kiparsky (1982)) account for conversion by assuming that a null affix is added to a base.

Example
they account for verbs such as to clean, to thin, to slow, to warm etc. by assuming that these have the structure [[...] A 0]V.

Links
Utrecht Lexicon of Linguistics