initiator

noun5

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