Polyalphabetic cipher
A cipher that switches substitution alphabets through the text, so plain frequency analysis loses its grip.
Leon Battista Alberti sketched the idea around 1467 with his cipher disk. Giovan Battista Bellaso formulated the keyword-driven version in 1553, where a key word decides which alphabet each letter is enciphered with. The cipher ended up carrying Vigenère's name, largely because of 19th-century attribution, even though he only described a closely related method.
It took three centuries to fall in print. Kasiski's 1863 method exposes the key length from repeated fragments in the ciphertext. Once the length is known, the text splits into several streams, and each one is a plain substitution cipher that frequency analysis handles easily.