Source: Rocket - Definition and More from the Free Merriam-Webster Dictionary

1
a : a firework consisting of a case partly filled with a combustible composition fastened to a guiding stick and propelled through the air by the rearward discharge of the gases liberated by combustion
b : a similar device used as an incendiary weapon or as a propelling unit (as for a lifesaving line)
2
: a jet engine that operates on the same principle as the firework rocket, consists essentially of a combustion chamber and an exhaust nozzle, carries either liquid or solid propellants which provide the fuel and oxygen needed for combustion and thus make the engine independent of the oxygen of the air, and is used especially for the propulsion of a missile (as a bomb or shell) or a vehicle (as an airplane)
3
: a rocket-propelled bomb, missile, projectile, or vehicle