A judgment, based on the preponderance of the evidence, that a disclosure of information to unauthorized persons or a violation of the security policy for a system in which unauthorized, intentional or unintentional disclosure, modification, destruction, or loss of an object has occurred.
Sources:
CNSSI 4009-2015

  The disclosure of classified data to persons not authorized to receive that data.
Sources:
CNSSI 4009-2015 from NSA/CSS Manual Number 3-16 (COMSEC)

  The unauthorized disclosure, modification, substitution, or use of sensitive data (e.g., keys, metadata, or other security-related information) or the unauthorized modification of a security-related system, device or process in order to gain unauthorized access.
Sources:
NIST SP 800-152 under Compromise (noun)

  To reduce the trust associated with a key, its metadata, a system, device or process.
Sources:
NIST SP 800-152 under Compromise (verb)

  Disclosure of information to unauthorized persons, or a violation of the security policy of a system in which unauthorized intentional or unintentional disclosure, modification, destruction, or loss of an object may have occurred.
Sources:
NIST SP 1800-25B under Compromise
NIST SP 1800-26B under Compromise

  The unauthorized disclosure, modification, substitution, or use of sensitive information (e.g., a secret key, private key or secret metadata).
Sources:
NIST SP 800-57 Part 2 Rev.1 under Compromise

  The unauthorized disclosure, modification, substitution, or use of sensitive data (e.g., a secret key, private key, or secret metadata).
Sources:
NIST SP 800-175B Rev. 1 under Compromise

  The unauthorized disclosure, modification, substitution or use of sensitive key information (e.g., a secret key, private key, or secret metadata).
Sources:
NIST SP 800-57 Part 1 Rev. 5 under Compromise

  The unauthorized disclosure, modification, or use of sensitive data (e.g., keying material and other security-related information).
Sources:
NIST SP 800-133 Rev. 2 under Compromise