A line joining points with the same surface brightness on a plot or in image of a celestial object such as a nebula or galaxy.
Isophote, from → iso- + a combining form of Gk. phos (gen. photos) "light."
Izošid, from izo-, → iso-, + šid "light, sunlight," from Mid.Pers. šêt "shining, radiant, bright;" Av. xšaēta- "shining, brilliant, splendid, excellent."