A deflector pulse is an energy discharge which could be produced by a starship's main navigational deflector. Created by setting a vessel's deflector beam to a narrow pulse, it could be used for a variety of purposes, including overloading an enemy ship's weapons systems, reinforcing shields, and as a means of communication.
In 2154, the crew of the Enterprise NX-01 used a deflector pulse to disable the interspatial manifold on the Delphic Expanse Sphere designated "Sphere 41", setting off a chain reaction which destroyed the entire Sphere network. (ENT: "Zero Hour")
In 2373, the crew of the USS Voyager initiated a deflector pulse composed of polarons to counter the attack of the Federation timeship Aeon, whose pilot, Captain Braxton, was attempting to destroy Voyager with a subatomic disruptor. Braxton managed to remodulate his disruptor, causing Voyager's deflector to lose power. Captain Janeway then ordered the deflector adjusted to match the frequency of the disruptor, an act which successfully overloaded Braxton's emitter. (VOY: "Future's End")
A year later, while piloting the Delta Flyer II, Harry Kim used a deflector pulse to disable the weapons systems of an Annari vessel. Setting the deflector beam to a narrow pulse of 0.4 microns, Kim fired as soon as the Annari weapons were fully-charged, triggering an overload in their phaser banks. (VOY: "Nightingale")
In 2378, The Doctor targeted a deflector pulse, generated from the Delta Flyer, at Voyager's transceiver array, and covertly accessed the ship's holographic database. Downloading Captain Janeway's physical parameters, he then realigned his holomatrix to disguise himself as her, and, using the pulse, communicated with the Voyager crew in an attempt to deceive them into ejecting the warp core. (VOY: "Renaissance Man")