Can we say it's hybridisation if it's the same species?
The question boils down to the definition of species and hybridization.

In general, we use the term hybridization to talk about mating between individuals from different species but really the term species does not mean as much as most people would think it does. Some people may use the term hybridization to talk about mating between lineages that are usually considered belonging to the asme species.

Have a look at How could humans have interbred with Neanderthals if we're a different species? for more information on the problems of definition of species.

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Here are terms and concepts that are related and may help you


gene flow
introgression
interspecific mating
secondary contact
partial reproductive isolation