A quadratic function is a noun. The plural form would be quadratic functions.
"Hearty" can function as an adjective.
"Congregation" can function as a noun.
Function can be a noun and a verb. Noun: What something is used for. Verb: To have a function.
The word "conceive" can function as a verb.
"Overdue" can function as an adjective or an adverb.
The word "terrestrial" can function as an adjective.
The word "perspective" can function as a noun.
"Acid" can function as a noun or an adjective.
"Forth" can function as an adverb or a preposition.
The word "ledge" can function as a noun.
The word "assure" can function as a verb.
The word "beach" can function as a noun.