A blackout is when electric power suddenly stops working across a neighborhood, city, or even larger area, leaving homes, schools, and businesses without lights, heat, or anything else that runs on electricity. During a blackout, refrigerators stop humming, traffic lights go dark, and the familiar glow of streetlights disappears. Blackouts can happen when storms damage power lines, when too many people use electricity at once on a hot summer day (overloading the system), or when equipment at a power plant fails.

The word also describes losing consciousness for a brief time, like when someone blacks out from standing up too quickly or from not eating enough. Doctors might ask if a patient experienced a blackout to understand what happened during an injury.

In a completely different context, a blackout can mean blocking information from being shared, like a news blackout where authorities prevent reporters from covering a story, or a media blackout during a sensitive military operation.

During wartime, a blackout historically meant turning off all lights at night so enemy aircraft couldn't see cities to bomb them. In World War II, entire cities would go dark after sunset, with people covering their windows with heavy curtains and streetlights turned off completely.