The definition of hybrid is "the offspring of two plants or animals of different species or varieties, such as a mule (a hybrid of a donkey and a horse)."
There are a number of different hybrids that exist in the cat world, both in the wild and domestically. However, the term "hybrid" has recently come to be applied most commonly with domestic × wild hybridization.
Domestic × wild hybridization are cat breeds where the original lineage / ancestry of the breed can be traced to crossbreeding between a domestic cat and a specific breed of wild cat, such as Savannahs (a hybrid of a serval and a domestic cat).
These cat breeds have the loving nature of domestic cats yet require more considerations for play, training and care due to their lineage.