It can vary a bit. Orthodox on the first move for white would be 1.e4, d4, c4, Nf3 or g3. They're all moves that occupy or control the center (or plan to in the case of g3). 1.b3 1.Nc3 and 1.f4 are probably fine too, but may be a gray area for some (I guess we'd need some stats).
More broadly it would be any opening that doesn't follow opening principals of the center and development and as a trade off hopes novelty and traps will provide an advantage after their opponent doesn't find the best replies.
Those that allow quick equality for black or an early small lasting advantage for white are probably debatable. Usually an opening labeled as unorthodox will give a bit more of an advantage for the sake of novelty, say, half a pawn. One metric for this kind of opening would be to look at a large database of master games where any opening that doesn't appear (or is very rare) could be considered unorthodox.
I've seen this name been givnin to a couple of openings and openings being called this and im not wuite sure why is it jsut because there lessplayed than other openings ?