A bomb was an explosive weapon.
On Hedgewick's World of Wonders, the platoon stationed there possessed a bomb that could destroy a planet with orders to use it if Cybermen were encountered an unable to be defeated. Initally the Doctor was against using it, but after learning that there were three million Cybermen, he had Ludens Nimrod Kendrick Cord Longstaff XLI, the Emperor who was better known as Porridge activate it. The planet was destroyed along with all the Cybermen though one Cybermite survived. (TV: Nightmare in Silver)