Definition of runaway reaction in US English:

runaway reaction

noun

  • 1A disorder found in troubled adolescents who perceive running away to be a solution to difficulties in the home, at school, etc.

  • 2A chemical or nuclear reaction that is accelerated by the products of the reaction or its effects (such as a rise in temperature), so that it becomes uncontrollable and continues until there are no reactants left.

Origin

1930s.