The Collaborative International Dictionary of English v.0.48:

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.
   [PJC]

WordNet (r) 3.0 (2006):

Down syndrome
    n 1: a congenital disorder caused by having an extra 21st
         chromosome; results in a flat face and short stature and
         mental retardation [syn: mongolism, mongolianism,
         Down's syndrome, Down syndrome, trisomy 21]