Event

Any observable occurrence in an information system. 

See the following under Event for more information:  
NIST SP 800-53 Rev. 4. 
NIST SP 800-61 Rev. 2. 
NIST SP 800-61 Rev. 2. 

Something that occurs within a system or network. 

See  NIST SP 800-92 under Event for more information. 
 
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). 

See NISTIR 8183 under Event (Framework for Improving Critical Infrastructure Cybersecurity, version 1.0) for more information.