A quasi-stellar radio source is better known as a quasar. The word is a shortened form of “quasi-stellar [star-like] radio source,” a name coined because these objects looked like faint stars in optical images while emitting unusually strong radio waves.
Quasars are extremely luminous active galactic nuclei. Their energy comes from matter spiraling into a supermassive black hole at the center of a galaxy. The accretion disk becomes intensely hot, producing radiation across the electromagnetic spectrum; some quasars also launch powerful jets of particles.
Astronomers first encountered quasars as puzzling radio sources in the 1950s. Their star-like appearance and strange spectral lines initially made their nature uncertain. Measuring their large redshifts eventually showed that they are immensely distant and extraordinarily powerful galactic cores.
Not every quasar is strongly radio-emitting today. “Quasi-stellar object,” or QSO, is a broader modern term, while quasar remains the familiar canonical name for the class.