initiator

noun

1. One who initiates.
2. A substance that initiates a chain reaction or polymerization.
3. 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.’”

4. A task (in a mainframe computer) that initiates multiple jobs.
5. A material whose presence in the body eventually leads to cancer.