A C in a circle is a copyright mark, also known as a copyright mark.

A mark indicating that a work is protected by copyright. The internationally accepted copyright mark stipulated in the Universal Copyright Convention is to use the first letter of Copyright in English followed by a circle to indicate that the work is protected by copyright. Records, audiovisual products, etc. use the symbol (the first letter of the English record phonogram followed by a circle) to indicate that the work is protected by copyright. The copyright mark should be prominently displayed on the work. However, in most countries, copyright protection for works does not depend on whether there is a copyright mark, and our country does not mandate that works with a copyright mark be protected.