A power source was an object's source of power.
In 2152, while Malcolm Reed, Trip Tucker, and Travis Mayweather examined a 31st century Earth vessel, Mayweather did not see anything that looked like a power source. (ENT: "Future Tense")
In 2259, Una Chin-Riley decided to provide a dedicated power source direct from the USS Enterprise's warp core to Doctor Joseph M'Benga's medical transporter so that he could maintain his cygnokemia-diagnosed daughter Rukiya in the pattern buffer and ensure that the medical transporter would not inhibit the ship's biofilters again. (SNW: "Ghosts of Illyria")
In 2268, Montgomery Scott speculated that a power source beneath the surface of Sigma Draconis VI was either a nuclear pile 100 miles across or ion power. According to Scott, it was enough power to push the planet out of orbit. (TOS: "Spock's Brain")
In 2365, on the planet Iconia, Lieutenant Commander Data detected a vast underground power source which was controlled by a console. Captain Jean-Luc Picard detonated the power source by launching Iconian probes. (TNG: "Contagion")
In 2370, Martus Mazur wasn't sure how to turn off his gambling devices and he believed they had some kind of internal power source. Captain Benjamin Sisko and Lieutenant Jadzia Dax then proceeded to destroy the gambling devices by shooting them with phasers. (DS9: "Rivals")