Found:

null affixation

MORPHOLOGY: 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.

LIT.	Bloomfield (1933) Kiparsky, P. (1982)