filter vs air filter

filter and air filter both are nouns.

Word	Noun	Adjective	Verb	Adverb
filter	Yes	No	Yes	No
air filter	Yes	No	No	No

As nouns, air filter is a hyponym of filter; that is, air filter is a word with a more specific, narrower meaning than filter:

filter (noun)	air filter (noun)
device that removes something from whatever passes through it	a filter that removes dust from the air that passes through it
an electrical device that alters the frequency spectrum of signals passing through it

filter (verb)	air filter (verb)
remove by passing through a filter
pass through
run or flow slowly, as in drops or in an unsteady stream