The Collaborative International Dictionary of English v.0.48:

centromere \centromere\ n. (Biol.)
   a specialized condensed region of a chromosomes that appears
   during mitosis where the chromatids are held together to form
   an X shape.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

centromere
    n 1: a specialized condensed region of each chromosome that
         appears during mitosis where the chromatids are held
         together to form an X shape; "the centromere is difficult
         to sequence" [syn: centromere, kinetochore]