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A certificate subject which uses its private key for purposes other than signing certificates.
NIST SP 800-15
Relying Parties and Subscribers.
NIST SP 800-32 under End Entity
An entity that is identified as the subject of a certificate at the end of a certification path or shares a symmetric key with other entities for communication.
NIST SP 800-57 Part 2 Rev.1 under End entity