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):

mongolism
    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]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "mongolism":
   Christmas disease, Hartnup disease, Werdnig-Hoffmann disease,
   achromatic vision, albinism, amentia, arrested development,
   backwardness, blithering idiocy, color blindness, cretinism,
   cystic fibrosis, dichromatic vision, dysautonomia, half-wittedness,
   hemophilia, ichthyosis, idiocy, idiotism, imbecility, infantilism,
   insanity, mental defectiveness, mental deficiency, mental handicap,
   mental retardation, mongolianism, mongoloid idiocy, moronism,
   moronity, mucoviscidosis, muscular dystrophy, pancreatic fibrosis,
   profound idiocy, retardation, retardment, sickle-cell anemia,
   simple-wittedness, simplemindedness, simpleness, simplicity,
   subnormality, thalassemia