: an attempt to gain illegal access to a computer or computer system for the purpose of causing damage or harm
An undermine-first strategy allows the United States government to intrude into adversaries' systems, exploiting this covert access to gather intelligence and occasionally to deliver a destructive cyberattack. Joshua A. Kroll
If your data is valuable enough, there is almost nothing you can do to provide total security against an expert adversary. Simply put, the attacker may be smarter than anyone you have defending the network. Cyberattacks are not only impossible to block, they're often difficult to detect; you may not even know you're under attack in the first place. Robert W. Lucky