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.
A lord, master, or other person in authority, most commonly used vocatively: formerly in speaking to elders and superiors, later only when addressing a sovereign.
A male animal that has fathered a particular offspring (especially used of domestic animals and/or in biological research).
to father; to beget.