A quasar is a specific type of astronomical object formed by matter falling into a super-massive black hole. The resulting energy is released as X-rays in all directions. However, the X-ray release is most intense directly above or below the galactic plane. Observed from this view, the resulting bright object is known as a Quasar.
During the original five-year mission of the USS Enterprise (NCC-1701), Captain James T. Kirk was under standing orders to investigate all quasars, and quasar-like phenomena. In 2267, the Enterprise encountered Murasaki 312, a quasar-like electromagnetic phenomenon. (TOS: "The Galileo Seven")
In 2376, Tuvok described Kelemane's planet as having a high rate of rotation "like a quasar". (VOY: "Blink of an Eye") Also, Icheb informed Seven of Nine that when he was on the Borg cube: "never thought about what was outside, such as pulsars, quasars, and nebulas...". (VOY: "Child's Play")