You use runaway to describe a situation in which something increases or develops very quickly and cannot be controlled. Our Grand Sale in June was a runaway success. a runaway best-seller
A runaway vehicle or animal is moving forward quickly, and its driver or rider has lost control of it. The runaway car careered into a bench, hitting an elderly couple. someone, especially a child, who has left home without telling anyone and does not intend to come back run away run