[ rē-ăk′tĭv ]
Thrust generated by an engine through the rapid expulsion of propellant or its exhaust in a particular direction. By Newton's third law, the force required to expel the propellant or exhaust is counterbalanced by an equal and opposite force, driving the engine (and its vehicle) in the opposite direction. See also ion engine Newton's laws of motion.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.