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"[1]; longer delays were intended to disrupt salvage and other activities, and spread terror, in areas where there could still be live bombs, and to attack bomb disposal workers.
Such bombs were used widely by British and German bomber aircraft during World War II. One use was to hamper and delay reconstruction and repair of bombed airfields.
Towards the end of the war both British and German bombs became de facto mines, with secondary fuze mechanisms activated by light, tilting, magnets (used by bomb disposal workers to try to disable a possible steel clockwork mechanism), etc., to kill those trying to disarm them.