This may just be arguing semantics to you but to me there is a very significant difference between the words species and race and the two aren't interchangeable. Species: Two lifeforms (animals, plants, etc.) that are genetically different to the point where they cannot interbreed viable offspring. Though plants have been known to make viable hybrids, animals by definition cannot. Animals that are closely related may be able to produce an offspring (mule, hinny, liger) but they cannot reproduce to create another of their kind. A mule cannot mate with another mule and produce a mule. Race: The human equivalent of animal's breed or plant's cultivar. Meaning they are of the same species and can produce viable offspring. A European may look different from an African, but they are both still human. My fantasy novel has a few sentient species and races that divide those species. I am not going to go out of my way to define the two in the narrative and explain the differences as it would be terrible writing, but I want to know if it would stump my readers at all at a casual reference, should it arise? Would it be at all a problem?