noun

• (Internet) An intentional outage of a website or other online service, typically as a form of protest.
  "the 2012 English Wikipedia blackout"
• (historical) The mandatory blocking of all light emanating from buildings, as well as outdoor and street lighting as a measure against aerial bombing or naval attack, as imposed during, e.g., World War II.
  "In co-operation with the Government departments, the British railway companies prepared their stations, offices, docks, hotels, trains, and other premises for the necessary blackout of lighting that it was realised would be required in a state of emergency, and in the case of the experimental voluntary blackouts which took place in July and August the railway companies concerned co-operated in every way possible."
• (attributive) The blocking out of as much light as possible.
  "blackout blinds; blackout curtains"
• (slang, criminology, rare) A mass murder committed, usually in an urban area, to eliminate potential witnesses of a previous crime.

verb

• (nonstandard) Alternative form of black out.
  "Scouts assisted Mabel Smythe Building officials in blackouting windows."