A segment of eukaryotic DNA consisting of highly-repeated nucleotide sequences which, when the entire duplex DNA molecule is denatured (the double-stranded helix comes apart and becomes single strands) and then allowed to renature (the complementary single strands come together to form a helix), will be the first segment to renature.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.