Technicaly, cannibalism means that you eat only your kind. In our world´s case, humans eating humans, dogs eating dogs, cows eating cows (lol).
In TES, it´s just all matter of interpretation. In TES cannibalism is categorized as eating sentient beings, which are most of the playable races. For example, Bosmer are called cannibals because they eat pretty much everyone, lol. Khajiit, Bosmer, Men, Mer...they don´t give a fuck. And in TES, that´s called cannibalism.
Interesting question arises when for example Dunmer, who consider Argonians, Khajiit and Orcs nothing but mere beasts. They use them as slaves, but would it be considered cannibalism, when they don´t consider these races..."human"? I say "human" because we know that in our world, many Empires in the past had slaves and they didn´t consider them to be humans, so it´s as close as I can get with my poor vocabulary.