Decay can function as both a noun and a verb. As a noun, it refers to the process of something breaking down or decomposing. As a verb, it describes the action of deteriorating or decomposing.
No part of "tooth decay" is a structure or function.
part of speech
The part of speech for this particular word is a noun.
The part of speech for "answer" is a noun.
adverb
what part of speech is beneath
The part of speech for "explicit" is an adjective.
The part of speech for "twirl" is a verb.
The part of speech for "unfamiliar" is an adjective.
The part of speech of "came" is a verb.
The part of speech for "lecturer" is a noun.
The part of speech for "indefinite" is an adjective.