|Definition - The use of a verb or an adjective as a noun.|
(1) Grammatical Nominalization (a grammatical expression is turned into a noun phrase):
"The recipe involved combining two ingredient" becomes "The recipe involved the combining of two ingredients."
(2) Lexical Nominalization (affixes are added to convert into a noun):
difficult becomes difficulty and careless becomes carelessness.
Etymology - The word ultimately derives from the Latin nominalis, pertaining to a name or names.
Oxford English Dictionary - The first citation for the word nominalize (to convert into a noun) is from 1659:
"Verbs … nominalized, do admit one termination familiarly, that suffer not another."
(Instructions Oratory 32)