"faint luminous ring caused by diffraction of light," 1660s, from Greek anthelion, noun use of neuter of anthelios, from assimilated form of anti "opposite" (see anti-) + hēlios "sun" (from PIE root *sawel- "the sun").

"the luminous ring produced by pressing the eyeball with the finger, etc.," 1872, from French phosphène, irregularly formed from Greek phōs "light" + phainein "to show," both from PIE root *bha- (1) "to shine."

1964, from "quas(i-stell)ar radio source" (1963); see quasi- + stellar. So called because they resembled stars in photographic images; now thought to be massive, distant, extremely luminous active galactic nuclei.

1660s, "partially shaded region around the shadow of an opaque body, a partial shadow," from Modern Latin penumbra "partial shadow outside the complete shadow of an eclipse," coined 1604 by Kepler from Latin pæne "nearly, almost, practically," which is of uncertain origin, + umbra "shadow" (see umbrage). Figurative use is by 1801. Related: Penumbral.

All points within the penumbra are excluded from the view of some part of the luminous body, and are thus partially shaded; while all points within the umbra, or total shadow, are completely excluded from view of the luminous body. [Century Dictionary]

also fire-fly, "An insect which has the faculty of becoming luminous" [Century Dictionary], 1650s, from fire (n.) + fly (n.). Ancient Greek had kysolampis "firefly, beetle with a tail that lights up at night," from kysos, kysthos "buttocks" + lampein "to shine."  

1550s, "somewhat dark, not luminous, dim;" see dusk + -y (2). "The normal source of an adj. in -y is a sb.; but the substantival use of dusk is not known so early as the appearance of dusky, so that the latter would appear to be one of the rare instances of a secondary adj. ..." [OED]. Meaning "rather black, dark-colored" is from 1570s. Related: Duskily; duskiness.

late 14c., "cloudy, misty, hazy" (of the eye, fire-smoke, etc.), from Latin nebulosus "cloudy, misty, foggy, full of vapor," from nebula "mist, vapor" (from PIE root *nebh- "cloud"). The figurative sense of "hazy, vague, formless" is attested by 1831. Astronomical sense, in reference to stars or star clusters surrounded by luminous haze, is from 1670s. Related: Nebulously; nebulousness.

"glowing with heat, rendered luminous by heat," 1794, from French incandescent (18c.) or directly from Latin incandescentem (nominative incandescens), present participle of incandescere "become warm, glow, kindle," from in- "within" (from PIE root *en "in") + candescere "begin to glow, become white," inceptive of candere "to glow, to shine" (from PIE root *kand- "to shine"). In reference to electric light, from 1881. The verb incandesce (1838), originally in science, is perhaps a back-formation.

"not dark," Old English leoht (West Saxon), leht (Anglian), "luminous, bright, beautiful, shining; having much light," from Proto-Germanic *leuhta- (source also of Old Saxon and Old High German lioht, Old Frisian liacht, German licht "bright"), from the source of Old English leoht (see light (n.)). Meaning "pale-hued" is from 1540s; prefixed to other color adjectives from early 15c. In earlier Middle English in reference to colors it meant "bright, vivid" (early 14c.).