A set of events which an observer believes to have information assurance consequences on some entity, the target of the attack. From the perspective of an administrator responsible for maintaining a system, an attack is a set of one or more events that has one or more security consequences. From the perspective of a neutral observer, the attack can either be successful, an intrusion, or unsuccessful, an attempted or failed intrusion. From the perspective of an intruder, an attack is a mechanism to fulfill an objective. It is unclear whether an unsuccessful attack is an intrusion.
Intrusion seems to imply forced entry, while attack seems to only imply the application of force.
Host xyz attacked site 123 with the blatsit attack. To begin to act upon destructively, to begin to destroy, expose, alter, or disable. Contrast with intrude. Contrast consequence with impact. For a given action, its consequence is the terminal state and its impact is the state transition from the starting system state to the terminal system state.