initiator

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initiator

[i′nish·ē‚ād·ər]
(chemistry)
The substance or molecule (other than reactant) that initiates a chain reaction, as in polymerization; an example is acetyl peroxide.
(computer science)
A part of an operating system of a large computer that runs several jobs at the same time, setting up the job, monitoring its progress, and performing any necessary cleanup after the job's completion.
(ordnance)
A device used as the first element of an explosive train, such as a detonator or squib, which upon receipt of the proper mechanical or electrical impulse produces a burning or detonating action; it generally contains a small quantity of a sensitive explosive.

initiator