double-decker vs autobus

double-decker and autobus both are nouns.

Word	Noun	Adjective	Verb	Adverb
double-decker	Yes	No	No	No
autobus	Yes	No	No	No

As nouns, double-decker and autobus are synonyms defined as:

double-decker and autobus: a vehicle carrying many passengers; used for public transport

Other synonyms of double-decker include bus, charabanc, coach, jitney, motorbus, motorcoach, omnibus, passenger vehicle.

double-decker (noun)	autobus (noun)
a vehicle carrying many passengers; used for public transport	a vehicle carrying many passengers; used for public transport