Why is Cyberterrorism difficult to define?

Why is Cyberterrorism difficult to define? Many cybersecurity professionals believe the likely application of “cyber terrorism” to be an asymmetric attack against some portion of this nation’s critical infrastructure. Which critical infrastructure do you think to be a likely target and why? Who should be responsible for protecting that infrastructure, and why? Would this vary based on whom the attacker is—a state actor, a non-government organization, or an individual?