initiator

One who initiates.

A substance that initiates a chain reaction or polymerization.

A component of a nuclear weapon that produces a burst of neutrons to start the chain reaction.

‘This, er, initiator of polonium and lithium, would it be used in an anti-personnel bomb?’ he asked. ‘Oh yes, you could say so, boyo,’ replied the Welshman. ‘An initiator, you see, is what sets off a nuke.’

A task (in a mainframe computer) that initiates multiple jobs.

A material whose presence in the body eventually leads to cancer.