double-decker vs motorbus

double-decker and motorbus both are nouns.

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

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

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

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

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