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 brawler or disputant.
Someone who wrangles or corrals.
A cowboy who takes care of tourists.
A student who has completed the third year of the mathematical tripos with first-class honours.
An animal handler or trainer.
A special education teacher.
A cowboy who takes care of saddle horses.