The term blackout in peacetime refers to a cessation of electrical energy through electric power transmission systems. In wartime it can also mean the minimizing of external light sources.
In general during peacetime, a blackout is an unintentional total electrical power outage. It is sometimes contrasted with the term brownout which is a reduction in power, and selected power outages, when demand exceeds supply (see rolling blackout).
However a blackout can be intentional, for instance collectively minimizing external light (during war), including upwards directed (or reflected) light, to "hide" from enemy aircraft (in the case of an occupied country: the enemy of the occupying force). Additionally, intentional blackouts can be used in coastal regions to prevent city lights from outlining ships (shore-side blackouts ). This makes ships hard for enemy submarines to detect.