As a proper noun apple
is a nickname for new york city, usually “the big apple”.
As a noun noun is
(grammar|sensu lato) a name of a thing either a noun substantive, which can stand alone and does not require another word to be joined with it to show its signification, or a noun adjective, which can not stand by itself, but requires to be joined with some other word, in order to make sense.
As a verb noun is
to convert a word to a noun.