Hybrids are dolls that use different parts from different companies and/or different editions.
Basically, it is a doll not using all it's original parts it came with, whether it's from the same company or not.
This does not normally include common interchangeable parts like eyes and wigs etc., but actual doll parts like limbs, heads, and bodies, though some may consider those parts as part of making a hybrid since it is from a different source. In some cases, special parts would also be included like horns, wings, etc. as parts for a hybrid, especially if it originate from another doll or is not originally part of the original doll.