Down syndrome

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A congenital disorder caused by the presence of an extra 21st chromosome and marked usually by moderate to severe mental retardation, short stature, and a flattened facial profile.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Condition caused by a chromosomal excess, whereby the patients bear a certain resemblance to the Mongoloid race, such as a small head and tilted eyelids, and typically have a delay in cognitive ability and physical growth.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation