Down syndrome vs trisomy 21

Down syndrome and trisomy 21 both are nouns.

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

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

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

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

Down syndrome (noun)	trisomy 21 (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