What is the meaning of 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 trigger a fission chain reaction.

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

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

second/third-person singular future passive imperative of initiƍ

Source: wiktionary.org