An authorized key that is used from outside the organization (or outside the environment considered for SSH user key management purposes), or an identity key that is used for authenticating to outside the organization (or outside the environment considered for SSH user key management purposes). Key rotation can break external keys, and therefore it must be ensured that the other side of trust relationships involving external keys is also properly updated as part of rotation. Alternatively, rotation of external keys may be prevented, but that is not a sustainable solution long-term.
Sources:
NISTIR 7966