of three nucleotides repeated in tandem on the same chromosome a number of
times. A normal, polymorphic variation in repeat number with no clinical
significance commonly occurs between individuals; however, repeat numbers
over a certain threshold can, in some cases, lead to adverse effects on the
function of the gene, resulting in genetic disease.
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
Related discussion in the Handbook
See also Understanding Medical Terminology.