An occurrence (e.g., an auditable event or flag) considered to have potential security implications to the system or its environment that may require further action (noting, investigating, or reacting).
Sources:
CNSSI 4009-2022

  Any event that attempts to change the security state of the system, (e.g., change discretionary access controls, change the security level of the subject, change user password, etc.). Also, any event that attempts to violate the security policy of the system, (e.g., too many attempts to login, attempts to violate the mandatory access control limits of a device, attempts to downgrade a file, etc.).
Sources:
CNSSI 4009-2022 from DoD 5200.28-STD

  Actions taken through the use of an information system or network that result in an actual or potentially adverse effect on an information system, network, and/or the information residing therein.
Sources:
NIST SP 800-160 Vol. 2 Rev. 1 under cyber incident from CNSSI 4009-2022

  Any event that attempts to change the security state of the system (e.g., change access controls, change the security level of a user, change a user password).  Also, any event that attempts to violate the security policy of the system (e.g., too many logon attempts).
Sources:
NISTIR 5153 under Security Relevant Event from DoD 5200.28-STD