(1) An action initiated by the user or the computer.  For example, a user event would be any mouse movement or keystroke.  An internal event detected by software could be caused by myriad things from unexpected data in the input to a security camera sensing motion.  See 
event driven, 
event loop, 
event handler and 
event management system.
(2) A movie or video clip in the iMovie video editing application on the Mac.