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. A means used to confirm the identity of a station, originator, or individual.