ballista vs bolt thrower

ballista and bolt thrower both are nouns.

Word	Noun	Adjective	Verb	Adverb
ballista	Yes	No	No	No
bolt thrower	Yes	No	No	No

As nouns, ballista and bolt thrower are synonyms defined as:

ballista and bolt thrower: ancient missile weapon that throws bolts or stones

ballista (noun)	bolt thrower (noun)
ancient missile weapon that throws bolts or stones	ancient missile weapon that throws bolts or stones