flyover vs overpass

flyover and overpass both are nouns.

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

As nouns, flyover and overpass are synonyms defined as:

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

flyover (noun)	overpass (noun)
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
bridge formed by the upper level of a crossing of two highways at different levels