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 Hadley that he had prepared the jumpers, and ordered him 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")

In the final draft script of "The Naked Time", only one device was set up in the Jefferies tube by Scott, and instead of it being designated as a "jumper" (so-called by Scott in on-screen dialogue), it was referred to firstly, in a scene description, as a "Star Trek style jump wire" and then, in Scott's dialogue, as a "jump wire line".