A variant of DES encryption in which the basic algorithm is applied three times, with an encryption step, followed by a decryption step and then another encryption step. The first and third encryption steps use the same key, while the middle step uses a different one. Can be used in a compatibility mode where the two keys are the same so only the last encryption step is effective, turning it into normal DES.
Developed as a way to get around weaknesses (real or suspected) in DES, it's really only useful to take advantage of an existing DES implementation (e.g. system library or hardware chip). More modern encryption algorithms like AES offer at least as good security, and are faster.