nebula |ˈnebyələ|noun ( pl. -lae |-lē| or -las )1 Astronomy a cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch or as a dark silhouette against other luminous matter.• (in general use) any indistinct bright area in the night sky, for example, a distant galaxy.2 Medicine a clouded spot on the cornea causing defective vision.ORIGIN mid 17th cent. (as a medical term): from Latin, literally ‘mist.’