A delay-action bomb is an aerial bomb designed to explode some time after impact, with the bomb's fuzes set to delay the explosion for times ranging from very brief to several weeks. Short delays were used to allow the bomb to penetrate before exploding: "a delay action bomb striking the roof of a tall building will penetrate through several floors before bursting". A short delay would also prevent a fighter-bomber or ground-attack aircraft getting caught in the blast of its own bomb after a low-altitude attack. Longer delays were intended to disrupt salvage and other activities, to spread terror in areas where there could still be live bombs and to attack bomb disposal workers.
Towards the end of the war both British and German bombs became de facto mines, with a secondary fuze mechanism activated by light tilting or magnets to kill those trying to disarm them.