Simply, the measure of the power level in the core of a nuclear reactor as a percent of it's rated limit. No nuclear reactor is ever truly off. When a reactor is shutdown, they mean that the level of power is below the point of adding heat, usually around 10e-12% (.00000000001%). This is a very insignificant amount. The status of the power level is called the reactors criticality. If a reactor is sub-critical then power is decreasing. If a reactor is critical then power is constant. If a reactor is super-critical then power is increasing. If a reactor has reached prompt criticality then it has exploded by the time you realize it. Note: a supercritical reactor is NOT a bad thing. If reactor power is at .00000000001% and you want to raise it to, uh, say 30%. Then power must increase, therefore, the reactor must become super-critical. The media often likes to use these terms to confuse the uneducated masses.