[rossdavidh]

Arguably, Tolkien did say (in the Silmarillion) that orcs were corrupted elves, and then also said (in Unfinished Tales) that orcs were corrupted Wose. Then he also said that Saruman made orcs from corrupted Men.

Most plausibly, since there are numerous cases of different groups of orcs looking at least as different from one another as, say, Hobbits and Men and Wose and Dwarves, "orc" is just a term for any humanoid whose form has been corrupted by a Dark Lord.