A holocentric chromosome is a chromosome which has a centromere that, intsead of being focused at one specific point, appears to be spread along the length of the chromosome. During cell devision, spindle fibres attatch along the entire length of the chromosome. One species that has holocentric centromeres is Caenorhabditis elegans.
Hartl D.L. and Ruvolo M. (2012) Genetics analysis of genes and genomes, 8th edition, Burlington: Jones and Bartlett Learning p245