A type of heterochromatin (condensed and genetically inactivated portion of a chromosome) which forms at specific stages of an organism's life cycle. In contrast with constitutive heterochromatin (the other type of heterochromatin), facultative heterochromatin occurs in only one of the two members of a homologous chromosome pair and is not permanent.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.