Actually, hybrid doesn't mean a cross between two species. It means a cross between two things which are different in some way. Only one allele ("gene") needs to be different between two animals for it to qualify as a hybrid mating, and some important hybridisations do involve only one gene, the genetics literature is full of hybridisation studies which involve only one gene.
Colloquially, the term hybrid often means "interspecific hybrid", but interspecific hybrids (hybrids between two species) are by no means the only kind. It is perfectly valid to call any cross a hybrid, as long as you're refering to some difference between the parents, no matter how small that difference is.
Feel free to grab a biological dictionary and look the term up
