A pulsar is a type of star, a rapidly rotating neutron star that sends out streams of electrons at nearly the speed of light along their magnetic poles. These subatomic particles emit electromagnetic radiation, including radio waves, visible light, X-rays, and gamma rays, such as that as the pulsar rotates they appear to "pulse" with light to an observer.
A pulsar of 4.356 solar masses is located in the Lonka Cluster. When Captain Jean-Luc Picard was replaced by aliens in 2366, the doppleganger ordered the Enterprise dangerously close to that pulsar, to investigate the nature of authority. (TNG: "Allegiance")
In 2374, the USS Voyager discovered a binary pulsar system. Captain Kathryn Janeway later ordered Voyager to fly between the pulsars, forcing a Srivani research team to retreat from the ship but nearly destroying Voyager in the process. (VOY: "Scientific Method")
Derran Tal detected a pulsar field from the Varro generational ship in 2375. However, it was located near an inhabited star system, and so she was unable to explore it due to the general xenophobia amongst the Varro. (VOY: "The Disease")
Later that year, Reginald Barclay targeted a tachyon beam from the MIDAS array at a class B itinerant pulsar, to create an artificial wormhole that allowed Starfleet to contact Voyager. (VOY: "Pathfinder") The pulsar allowed two-way communications once every 32 days. (VOY: "Life Line")