The substance or molecule (other than reactant) that initiates a chain reaction, as in polymerization; an example is acetyl peroxide.

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.

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.