[a] data object uniquely referring to a specific identity of a protocol entity or individual in some context. . . . Identifiers can be based upon natural names — official names, personal names, and/or nicknames — or can be artificial (for example, x9z32vb). However, identifiers are by definition unique within their context of use, while natural names are often not unique.
"An identifier points to an individual. An identifier could be a name, a serial number, or some other pointer to the entity being identified. Examples of personal identifiers include personal names, Social Security numbers (SSNs), credit card numbers, and employee identification numbers. It is sometimes necessary to distinguish between identifiers and the things that they identify."