A pass-phrase, personal identification number (PIN), biometric data, or other mechanisms of equivalent authentication robustness used to protect access to any use of a private key, except for private keys associated with System or Device certificates.
Source(s):
CNSSI 4009-2015
from
CNSSI 1300
Private data, other than keys, that are required to access cryptographic modules (i.e., unlock private keys for signing or decryption events).
Source(s):
NIST SP 800-32
under Activation Data