Other forms: piracies
Piracy is a what pirates do: they steal stuff. If you plagiarize by stealing someone's words or ideas that’s piracy. Argh!
The original sense of piracy comes from pirates — thieves who hijack other boats, taking what they please. That should help you remember that piracy means to steal someone else's work, usually their words or ideas. A writer who doesn't cite their sources is committing piracy. Piracy is plagiarism, and it is legally and morally wrong.