round-trip ticket vs return ticket

round-trip ticket and return ticket both are nouns.

Word	Noun	Adjective	Verb	Adverb
round-trip ticket	Yes	No	No	No
return ticket	Yes	No	No	No

As nouns, round-trip ticket and return ticket are synonyms defined as:

round-trip ticket and return ticket: a ticket to a place and back (usually over the same route)

round-trip ticket (noun)	return ticket (noun)
a ticket to a place and back (usually over the same route)	a ticket to a place and back (usually over the same route)