Different species...cannot interbreed and create fertile offspring.
There's nothing wrong with this definition as far as it goes. It does have the consequence that there are 3 groups of entities: X, Y, and Z such that X and Y can produce fertile offspring; Y and Z can produce fertile offspring; but X and Z cannot. (I first heard of this with plants about 50 years ago. There are some animal examples.)
One gets the interesting observation that x&y belong to the "same" species; y&z belong to the same species; but x&z do not. Specieshood is not an equivalence relation.
Quantitive questions do remain. Horses and donkeys generally produce infertile offspring. Do the few fertile mules and hinnies make horses and donkeys the same species.