initiator

Short definition from Wiktionary

Noun

1. 1
   One who initiates.
2. 2
   A substance that initiates a chain reaction or polymerization.
3. 3
   A component of a nuclear weapon that produces a burst of neutrons to start the chain reaction.
4. 4
   A task (in a mainframe computer) that initiates multiple jobs.
5. 5
   A material whose presence in the body eventually leads to cancer.