An opaque unguessable subscriber identifier generated by a CSP for use at a specific individual RP. This identifier is only known to and only used by one CSP-RP pair.
Sources:
NIST SP 800-63-3
A meaningless but unique number that does not allow the RP to infer anything regarding the subscriber but which does permit the RP to associate multiple interactions with the subscriber’s claimed identity.
Sources:
NIST SP 800-63-3