While speciation is an important concept, especially with regard to evolutionary theories, I don't think there is a precise idea of species, and the value of the idea in measuring the similarity between types of animals is dubious.
If one of two previously interbreeding subspecies populations undergoes a mutational shift, where a single mutation catches on in it, such that the populations will find it difficult to interbreed, are the two populations necessarily as different as any two subspecies of the same species are to each other? Something slight change to A has finally tipped the scales and now B can freely mix with it any more. And could some closely related pairs of species (that have diverged from each other tens of thousands of years ago) be much more genetically similar than A and B are to each other?
The ability for two species to hybridize in vitro and produce functional offspring is one measure of their similarity. The ability for two species to form chimeras is yet another, more distant, measure, but an important one. Chimeras require embryonic cells that follow essentially the same body plan along with sharing important biochemical compatibilities.
Hybrids themselves test the boundaries of species. We are told now that only one wild canid was the ancestor of the domesticated dog, but some species of coyote and wolf reflect considerable gene flow between what are considered separate species. Some wild coyotes were determined to have more than half wolf ancestry. Some scientists seem to speculate that the some of this is due to coydogs mating with wolves, but I don't believe that accounts for all of it. At any rate, if some dogs have coyote blood, and some are part jackal, have they merely found evidence that most dog breeds have exclusive wolf ancestry? And do wolves have exclusive wolf ancestry?
There are also species of frogs and lizards that are not independent species but require repeated interbreeding with one or more of its parent species to keep the line going.
And there is ongoing speculation that in order to resolve certain discrepancies in the molecular clocks of human and chimp DNA we must allow that the proto-humans mated with proto-chimps long after the species had become separate species. This was apparently a desparate act at one of the several apparent severe genetic bottlenecks that the human species has endured. And of course, a small percentage of Europeans supposedly share some heritage with the Neanderthal subspecies of human, which was apparently not interbreeding with the rest of humanity so much.
Then there are various subspecies of squirrels around a national park such that subspecies A can breed with B, B with C, C with D, ... and E with F. But A can't breed with F. Take out all the subspecies in the middle and A and F suddenly are different species, even though there is gene flow thoughout the species, even between A and F, if only indirectly.