bus vs autobus

bus and autobus both are nouns.

Word	Noun	Adjective	Verb	Adverb
bus	Yes	No	Yes	No
autobus	Yes	No	No	No

As nouns, bus and autobus are synonyms defined as:

bus and autobus: a vehicle carrying many passengers; used for public transport

bus (noun)	autobus (noun)
a vehicle carrying many passengers; used for public transport	a vehicle carrying many passengers; used for public transport
a car that is old and unreliable
an electrical conductor that makes a common connection between several circuits
the topology of a network whose components are connected by a busbar

bus (verb)	autobus (verb)
remove used dishes from the table in restaurants
ride in a bus
send or move around by bus