A cipher is a secret code that disguises a message by systematically changing its letters or symbols. When you write in cipher, you transform normal text into something unreadable to anyone who doesn't know the key.

The simplest cipher might replace each letter with the one that comes three places later in the alphabet, so “HELLO” becomes “KHOOR.” Julius Caesar used this type of cipher two thousand years ago to send military messages his enemies couldn't read. More complex ciphers shuffle letters in intricate patterns or use completely different symbols.

During World War II, code breakers worked frantically to crack enemy ciphers, knowing that reading secret messages could change the outcome of battles. The Enigma machine created incredibly complex ciphers that took brilliant mathematicians years to solve.

Today, computers use mathematical ciphers to protect everything from text messages to bank accounts. When you see a little padlock icon in your web browser, cipher technology is scrambling your information so thieves can't steal it.

The word can also describe a person who seems to have no personality or importance, someone so quiet or unremarkable they seem like a zero. But this meaning is much less common than the code-related definition.