A cipher is the numeral zero or someone without influence or importance. Don't vote your popular classmate to be the student council leader if he or she is actually a cipher, someone who can't do the job effectively.
Cipher comes from the Arabic sifr meaning "nothing" or "zero" and came to Europe as a word 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 enabling you to understand the message. An encoded message is also called a cipher. Send one to your friend if you don't want anyone to know what you're saying!