bubble vs air bubble

bubble and air bubble both are nouns.

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

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

Other hyponyms of bubble include foam, froth, soap bubble.

bubble (noun)	air bubble (noun)
a dome-shaped covering made of transparent glass or plastic	a bubble of air
an impracticable and illusory idea
a speculative scheme that depends on unstable factors that the planner cannot control
a hollow globule of gas (e.g., air or carbon dioxide)

bubble (verb)	air bubble (verb)
expel gas from the stomach
form, produce, or emit bubbles
cause to form bubbles
rise in bubbles or as if in bubbles
flow in an irregular current with a bubbling noise