The inability of a hybrid to produce viable offspring. Whether an animal hybrid is sterile or fertile depends mainly on how closely its two parent species are related. For example, mules (which are hybrids between male donkeys and female horses) are usually sterile, while hybrids between domestic dogs and wolves are fertile (hybrids between wolves and foxes are also fertile, but hybrids between dogs and foxes are sterile).

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.