Occurrence or change of a particular set of circumstances.
Sources:
NIST SP 800-160v1r1 from ISO Guide 73

  Any observable occurrence in a network or system.
Sources:
CNSSI 4009-2015 from NIST SP 800-61 Rev. 2
NIST SP 800-61 Rev. 2 under Event

  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. See incident. See also event, security-relevant event, and intrusion.
Sources:
CNSSI 4009-2015 under cyber incident

  Something that occurs within a system or network.
Sources:
NIST SP 800-92 under Event

  Any observable occurrence in a network or information system.
Sources:
NIST SP 800-37 Rev. 2 from NIST SP 800-61 Rev. 2 - adapted

  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 4009-2015

  Any observable occurrence in a system.
Sources:
NIST SP 800-53 Rev. 5 from NIST SP 800-61 Rev. 2 - Adapted

  Any observable occurrence on a manufacturing system. Events can include cybersecurity changes that may have an impact on manufacturing operations (including mission, capabilities, or reputation).
Sources:
NISTIR 8183 under Event from NIST Cybersecurity Framework Version 1.1, NIST Cybersecurity Framework Version 1.0
NISTIR 8183 Rev. 1 under Event from NIST Cybersecurity Framework Version 1.1
NISTIR 8183A Vol. 1 under Event from NIST Cybersecurity Framework Version 1.1
NISTIR 8183A Vol. 2 under Event from NIST Cybersecurity Framework Version 1.1
NISTIR 8183A Vol. 3 under Event from NIST Cybersecurity Framework Version 1.1