A cypher is a message written in a secret code. Spies during World War II sometimes communicated using cyphers.
If you use a cypher to send your friend a message, it may take her a while to figure out what you're trying to tell her. Some cyphers substitute numbers or symbols for letters, and you need a key to decipher — or interpret — them. Another kind of cypher is an unimportant person who's blank or devoid of personality — you might call a lifeless character in a book a cypher. The word has an Arabic root, sifr, "zero, empty, or nothing."