A cipher is a secret code, usually one that's created using a mathematical algorithm. Sometimes the message written in code is itself called a cipher.
Cipher comes from the Arabic sifr, which means "nothing" or "zero." The word came to Europe along with the Arabic numeral system. As early codes substituted numerals for letters to hide the word’s meaning, codes became known as ciphers. If you crack a cipher, you break the code and can understand the message. From its "zero" meaning, cipher can also be used for a person who has no influence or importance in the world.