(OTP) A security system that requires a new passWord every time a user authenticates themselves, thus protecting against an intruder replaying an intercepted passWord. OTP generates passWords using either the MD4 or MD5hashingalgorithms. The equivalent term "S/Key", developed by Bellcore, is a trademark of Telcordia Technologies, so the name OTP is used increasingly. See RFC 1760 - "The S/KEY One-Time PassWord System" and {RFC 1938} - "A One-Time PassWord System". . (2000-01-31)