This is not an official grammatical term. Rather, I made it up to refer to three structures in our language that can step in and perform the roles of the noun: (1) nominal clauses, (2) infinitive phrases, and (3) gerundive phrases. All great writers routinely use these structures, though they will use a true noun when it gets the job done.
Here are examples of the three main noun substitutes:
The fact that you visited Grammar.com shows your interest in good writing. (Nominal clause acting as the subject of the sentence.)
He wanted to win the game. (Infinitive phrase used as a noun, the object of the transitive verb want.)
He perfected his technique of swinging the golf club. (Gerundive phrase serving as the object of the preposition of.)