trisomy 21 vs Down syndrome

trisomy 21 and Down syndrome both are nouns.

Word	Noun	Adjective	Verb	Adverb
trisomy 21	Yes	No	No	No
Down syndrome	Yes	No	No	No

As nouns, trisomy 21 and Down syndrome are synonyms defined as:

trisomy 21 and Down syndrome: a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation

Other synonyms of trisomy 21 include Down's syndrome, mongolianism, mongolism.

trisomy 21 (noun)	Down syndrome (noun)
a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation	a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation