The description of the word you requested from the astronomical dictionary is given below.
quasar, plural quasars; [English] quasi-stellar radio source
A quasar is a celestial body that at first sight looks like a star (i.e., like a small point of light), but that after closer investigation often turns out to be in the center of a galaxy. A quasar is far brighter than the rest of the galaxy, and can therefore be seen at much greater distances than the galaxy itself. Corrected for the distance, quasars are about 100 times brighter than the brightest galaxies.
Astronomers think that quasars are really large black holes that capture great amounts of material from the surrounding galaxy, and that that material emits much energy as it falls into the black hole, not just as visible light, but also as, for example, radio waves, which is where quasars were first discovered.
All quasars are far away, which means they existed long ago only. It seems that many galaxies were quasars for a while in their youth.