Vigenere
https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
The Vigenere cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It employs a form of polyalphabetic substitution.
First described by Giovan Battista Bellaso in 1553, the cipher is easy to understand and implement, but it resisted all attempts to break it until 1863, three centuries later. This earned it the description le chiffre indechiffrable (French for 'the indecipherable cipher'). Many people have tried to implement encryption schemes that are essentially Vigenere ciphers. In 1863, Friedrich Kasiski was the first to publish a general method of deciphering Vigenere ciphers.
Incorrect answers:
Caesar - Monoalphabetic cipher where letters are shifted one or more letters in either direction. The method is named after Julius Caesar, who used it in his private correspondence.
Atbash - Single substitution monoalphabetic cipher that substitutes each letter with its reverse (a and z, b and y, etc).
Scytale - Transposition cipher. A staff with papyrus or letter wrapped around it so edges would line up. There would be a stream of characters which would show you your message. When unwound it would be a random string of characters. Would need an identical size staff on other end for other individuals to decode message.