What is "autokey cipher"

An autokey cipher (also known as the autoclave cipher) is a cipher which incorporates the message (the plaintext) into the key. There are two forms of autokey cipher: key autokey and text autokey ciphers. A key-autokey cipher uses previous members of the keystream to determine the next element in the keystream. A text-autokey uses the previous message text to determine the next element in the keystream.

In modern cryptography, self-synchronizing stream ciphers are autokey ciphers.