Terminal speed, also known as terminal velocity, is the maximum speed of a falling object when it can no longer accelerate from the gravitational pull or any constant force. This is due to air resistance negating the force of gravity acting on the object.
Terminal speed also applies on objects sinking in fluids. In this case, the liquid's buoyancy (instead of air resistance) counteracts the force of gravity. Objects falling in a vacuum have no terminal speed as there is no resistance to counteract the force of gravity. The term is popularized by skydivers, who use it for free falling and parachuting.