For a key pair, the owner is the entity that is authorized to use the private key associated with a public key, whether that entity generated the key pair itself or a trusted party generated the key pair for the entity.
Source(s):
NIST SP 800-56B Rev. 2

  For a static key pair, the entity that is associated with the public key and authorized to use the private key. For an ephemeral key pair, the owner is the entity that generated the public/private key pair. For a symmetric key, the owner is any entity that is authorized to use the key.
Source(s):
NIST SP 800-57 Part 1 Rev. 5 under Owner (of a key or key pair)

  A key pair owner is the entity that is authorized to use the private key of a key pair.
Source(s):
NIST SP 800-89

  For a symmetric key (i.e., a secret key), the entity or entities that are authorized to share and use the key.
Source(s):
NIST SP 800-133 Rev. 2

  For an asymmetric key pair consisting of a private key and a public key, the owner is the entity that is authorized to use the private key associated with the public key, whether that entity generated the key pair itself or a trusted party generated the key pair for the entity.
Source(s):
NIST SP 800-133 Rev. 2

  A user who can modify the contents of an access control list.
Source(s):
NISTIR 5153

  An Informative Reference produced by the owner of the Reference Document.
Source(s):
NISTIR 8278