travel vs air travel

travel and air travel both are nouns.

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

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

travel (noun)	air travel (noun)
the act of going from one place to another	travel via aircraft
self-propelled movement
a movement through space that changes the location of something

travel (verb)	air travel (verb)
change location; move, travel, or proceed, also metaphorically
make a trip for pleasure
travel upon or across
undergo transportation as in a vehicle
undertake a journey or trip
travel from place to place, as for the purpose of finding work, preaching, or acting as a judge