overpass vs flyover

overpass and flyover both are nouns.

Word	Noun	Adjective	Verb	Adverb
overpass	Yes	No	No	No
flyover	Yes	No	No	No

As nouns, overpass and flyover are synonyms defined as:

overpass and flyover: bridge formed by the upper level of a crossing of two highways at different levels

overpass (noun)	flyover (noun)
bridge formed by the upper level of a crossing of two highways at different levels	a flight at a low altitude (usually of military aircraft) over spectators on the ground
	bridge formed by the upper level of a crossing of two highways at different levels