The sign language most commonly used in the U.S. is American Sign Language, sometimes called Ameslan or just Sign. Those not conversant in Sign may suppose that it's an invented form of communication like Esperanto or Morse code. It's not. It's an independent natural language, evolved by ordinary people and transmitted culturally from one generation to the next.
I had no idea Sign was a natural language. I've always thought that it was an invented language, like Esperanto.