What is Blackout?

Legal Definition
In broadcasting, the term blackout refers to the non-airing of television or radio programming in a certain media market.

It is particularly prevalent in the broadcasting of sports events, although other television or radio programs may be blacked out as well. Most blackout policies serve to protect local broadcasters (primarily regional sports networks) from competition by "out-of-market" networks that carry different teams, by only allowing viewers to watch non-national telecasts of teams within their designated markets (with television providers blacking out regional telecasts of teams that are outside of their market; in turn, encouraging viewers to purchase subscription-based out-of-market sports packages), and by allowing teams to black out national telecasts of games that are also being shown by a local broadcaster. By contrast, the blackout policies of the National Football League serve to encourage attendance to games instead—by only allowing them to be broadcast on television in a team's designated market if a certain percentage of their tickets are sold prior to the game.
-- Wikipedia
Legal Definition
1. when commercial power is lost over a large area. 2. a stopping of activity over a period of time.