The Atbash cipher is one of the oldest known substitution ciphers. It originated from the Hebrew alphabet and was used in ancient Jewish texts. The name comes from the first and last letters of the Hebrew alphabet: Aleph-Tav-Beth-Shin.
In the Latin alphabet version, A becomes Z, B becomes Y, and so on. Because each letter maps to exactly one other letter (and vice versa), the cipher is its own inverse - applying it twice returns the original text.