A detonator is a device used to trigger

Detonation

Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...

 an explosive device

Explosive device

An explosive device is device that relies on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy. Explosive devices have applications as demolition devices and as weapons in the military....

. Detonators can be chemically, mechanically, or electrically initiated, the latter two being the most common.

The commercial use of explosives uses electrical detonators or the capped fuse which is a length of safety fuse

Fuse (explosives)

In an explosive, pyrotechnic device or military munition, a fuse is the part of the device that initiates function. In common usage, the word fuse is used indiscriminately...

 to which an ordinary detonator has been crimp

Crimp (joining)

Crimping is joining two pieces of metal or other malleable material by deforming one or both of them to hold the other. The bend or deformity is called the crimp.- Uses :...

ed. Many detonators' primary explosive is a material called ASA compound.