cello vs violoncello

cello and violoncello both are nouns.

Word	Noun	Adjective	Verb	Adverb
cello	Yes	No	No	No
violoncello	Yes	No	No	No

As nouns, cello and violoncello are synonyms defined as:

cello and violoncello: a large stringed instrument; seated player holds it upright while playing

cello (noun)	violoncello (noun)
a large stringed instrument; seated player holds it upright while playing	a large stringed instrument; seated player holds it upright while playing