Atbash is a simple substitution cipher in Hebrew. It consists of substituting aleph (the first letter) for tav (the last), beth (the second) for shin (one before last), and so on, reversing the alphabet. A couple of words in the Book of Jeremiah, Leb Kamai and Sheshakh, are atbash for Kasdim (Chaldeans) and Babel respectively, probably written thus.

External Links

Atbash as discussed on The Beginner's Guide to Cryptography