1. A symbol or group of symbols, or a series of bits, selected or derived in a prearranged manner and usually inserted at a predetermined point within a message or transmission for the purpose of attesting to the validity of the message or transmission.
2. A letter, numeral, group of letters or numerals, or any combinations of these, attesting to the authenticity of a message or transmission.
3. In INFOSEC, means used to confirm the identity or eligibility of a station, originator, or individual.
Source: from Federal Standard 1037C and adapted from the Department of Defense Dictionary of Military and Associated Terms and from the National Information Systems Security Glossary and from the Department of Defense Dictionary of Military and Associated Terms