Real Dictionary

blackout
A blackout is a period of time during a war in which towns and buildings are made dark so that they cannot be seen by enemy planes.

blackout in American English
the extinguishing of all stage lights to end a play or scene
noun: the extinguishing or concealment of all visible lights in a city, military post, etc., usually as a precaution against air raids

blackout in British English
noun: the extinguishing or hiding of all artificial light, esp in a city visible to an enemy attack from the air
verb: to obliterate or extinguish (lights)

blackout in Electrical Engineering
noun: A blackout is a power loss affecting many electricity consumers over a large geographical area for a significant period of time.