A jumper was a type of device which boosted a starship's power.
In 2266, Chief Engineer Montgomery Scott set up a series of three jumpers in a Jefferies tube aboard the USS Enterprise, while the vessel was attempting to break free of a steadily dwindling orbit of planet Psi 2000. After descending from the Jefferies tube, Scott notified a crewman he had prepared the jumpers, and ordered the crewman to stand by until Scott gave him the signal to activate the devices. Scott then left but, shortly thereafter, the ship's orbit was able to be stabilized. (TOS: "The Naked Time")