A bomb is usually some form of container filled with explosives that is caused to explode by an igniter. The explosion bursts the container which creates a pressure wave and shatter which is meant to cause destruction where it exploded.
The word bomb comes from the Greek bombos, an onomatopoeic term with about the same meaning as bom.
Bomb clearance means neutralizing unexploded bombs.
To make the bomb explode, you have to trigger the igniter, which is usually done by a trigger, remote control or some form of sensor. The igniter ignites the explosive either instantly or with a delay. Delay is usually done with a clock or timeline.
Bombs originally consisted of a powder-filled iron bullet with an opening for a stub. Smaller bombs were dropped by hand, while larger ones were fired with mortars. Bombs of varying, often very primitive construction, sometimes also referred to as hell machines, have been used in attacks by anarchists, revolutionaries and perpetrators. Today, bombs are most associated with terrorist attacks.