1. An identifier within or attached to a set of data elements.
2. One or more characters that (a) are within or attached to a set of data elements and (b) represent information about the set, including its identification.
3. In communications, information within a message that is used to identify specific system parameters, such as the particular circuit with which the message is associated.
Note: Messages that do not relate to call control should not contain a label.
4. In programming languages, an identifier that names a statement.
5. An identifier that indicates the sensitivity of the attached information.
6. For classified information, an identifier that indicates (a) the security level of the attached information or (b) the specific category in which the attached information belongs.
Source: from Federal Standard 1037C