What is your source for the "correct" definition?
And by what authority does that source tell everyone else how to use the word?
There are lots of sources all over the internet. But here are quotes from three dictionaries that I have available in actual paper form:
Websters's New World College Dictionary, Third Edition, copyright 1988
hybrid: 1. the offspring produced by crossing two individuals of unlike genetic constitution; specif., the offspring of two animals or plants of different races, varieties, species, etc.
Webster's Enclyclopedic Unabridged Dictionary of the English Language, New Deluxe Edition, copyright 1996
hybrid, noun: the offspring of two animals or plants of different breeds, varieties, species, or genera, esp. as produced through human manipulation for specific genetic characteristics
hybrid, adjective: bred from two distinct races, breeds, varieties, species, or genera.
This dictionary also discusses synonyms and antonyms specific to that definition as an adjective:
Syn. hybrid, mongrel refer to animals or plants of mixed origin. Hybrid is the scientific term: hybrid corn; a hybrid variety of sheep. Mongrel, used originally of dogs to denote the offspring of crossings of different breeds, is now extended to other animals and to plants; it is usually deprecatory, as denoting mixed, nondescript, or degenerate breed or character: a mongrel pup. Ant. purebred, thoroughbred
The New College Latin & English Dictionary, copyright 1995
hybrida: hybrid, mongrel, half-breed
So the Romans used "hybrid" to mean animals or people that were a mix within species. The people publishing dictionaries in the 1980s and 1990s used "hybrid" to mean crosses within a species.
The people posting definitions on dictionary sites on the internet also have definitions that match or closely resemble the ones I typed up above.
The wikipedia article on "hybrid" discusses both kinds of hybrids (between species and within species).
https://en.wikipedia.org/wiki/Hybrid_(biology)
(For the two English dictionaries, I did not bother to type up the other meanings that would apply to things like hybrid cars. The Latin dictionary did not have any other meanings.)
If a word is defined by how people DO use it, hybrid DOES include crosses within a species.
If a word is defined by how people are SUPPOSED to use it, who gets to decide that?
Also, do you have any OTHER good term for a cross between breeds of chicken, or a cross between varieties of plant? Or any OTHER good term for what we usually mean by "hybrid vigor"? (The situation when plants or animals peform better than either parent because they are a cross between parents of two kinds within one species.)
