The human(s) responsible for the management of a given certificate.
Source(s):
NIST SP 800-57 Part 2 Rev.1

  The entity that is responsible for managing the certificate, including requesting, replacing, and revoking the certificate if and when required. The certificate owner is not necessarily the subject entity associated with the public key in the certificate (i.e., the key pair owner).
Source(s):
NIST SP 800-175B Rev. 1 under Owner of a certificate

  A human entity that is identified as the subject in a public key certificate or is a sponsor of a non-human entity (e.g., device, application or process) that is identified as the certificate subject.
Source(s):
NIST SP 800-57 Part 1 Rev. 5 under Owner (of a certificate)