bolt thrower vs ballista

bolt thrower and ballista both are nouns.

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

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

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

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