What is "trisomy 21 syndrome"

Down syndrome \Down" syn`drome\, Down's syndrome \Down's" syn`drome\, n. (Med.) A congenital disorder caused by an extra chromosome 21 in some or all cells. It causes a variable number and degree of abnormalities, including retarded growth, mental retardation, a short nose, prominent epicanthic folds on the eyelids, a protruding lower lip, and other physical features having varying degrees of deviation from the normal. Called also mongolism, trisomy-21 and trisomy 21 syndrome.