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.
One who publishes, especially books.
A system or component that allows other components (subscribers) to receive notifications of something, such as an event.