A block cipher created by Dr. Ross Anderson, Dr. Eli Biham and Dr. Lars Knudsen as an entry for the NIST Advanced Encryption Standard contest, and was the second place candidate (59 votes to Rijndael's 86). The cipher is a SP network with 32 rounds and (like all the AES candidate algorithms) a 128-bit block size. It supports only a 256-bit key; it sort of cheated on the AES requirements by simply padding the 128- and 192-bit keys to 256 bits. It was the AES candidate that had the widest security margin among all the finalist algorithms but was also the slowest.
Specifications and GPLed reference code can be found at the Serpent home page at http://www.cl.cam.ac.uk/~rja14/serpent.html