Cease and desist

It arrives as a letter, usually from a solicitor, telling you to stop and setting a deadline. By itself it carries no force. No court has ruled on anything, and the sender is describing what they intend to do if you carry on. Creators tend to meet one over borrowed footage, a channel name close to somebody's trademark, or a hard claim made about a product on camera. There are 2 responses that reliably make things worse: ignoring it, and posting it for the audience to pile on.