There is a firm definition of species that works most of the time, but has been expanded with a lot of complexity and exceptions like rings of species for example.

In short if two animals can mate and produce fertile offspring similar to the parents, they are in the same species,

This means that all extant humans are in the same species (even if some are hybrids and some were isolated for a lot of time. Even if they never want to mate by religious reasons, geography or personal preferences)

Of course things can get messed fast. A tomato with a codfish genus included by humans to stand frost, is not a codmato suddently.