The Collaborative International Dictionary
Crepuscular \Cre*pus"cu*lar\ (-k?-l?r), Crepusculous \Cre*pus"cu*lous\ (-l?s), a. [Cf. F. cr['e]pusculaire.]
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Pertaining to twilight; glimmering; hence, imperfectly clear or luminous.
This semihistorical and crepuscular period.
--Sir G. C. Lewis. -
(Zo["o]l.) Flying in the twilight or evening, or before sunrise; -- said certain birds and insects.
Others feed only in the twilight, as bats and owls, and are called crepuscular.
--Whewell.
Douglas Harper's Etymology Dictionary
figurative use from 1660s; literal use from 1755, from Latin crepusculum "twilight, dusk," from creper "dusky," which is of unknown origin. Especially of evening twilight.
Wiktionary
a. 1 Of or resembling twilight; dim. 2 (context zoology English) active at or around dusk, dawn or twilight.
WordNet
adj. like twilight; dim; "the evening's crepuscular charm"
Wikipedia
Crepuscular animals are those that are active primarily during twilight (i.e., the period immediately after dawn and that immediately before dusk). It is thus to be distinguished from diurnal and nocturnal behavior where an animal is active during the hours of daylight or the hours of darkness respectively. The term is not precise, however, as some crepuscular animals may also be active on a moonlit night or during an overcast day. The term matutinal is used for animals that are only active before dawn, and vespertine for those only active after sunset.
The time of day an animal is active depends on a number of factors. Predators need to link their activities to times of day at which their prey is available, and victims try to avoid the times when their principal predators are at large. The temperature at midday may be too high or at night too low, so for many varied reasons, crepuscular activity may best meet their requirements by compromise. Some creatures may adjust their activities depending on local competition.
Usage examples of "crepuscular".
The remaining crepuscular rays through the heavy clouds spotlit the distant plain.
A minute more, and the flier was climbing the heavens along the crepuscular curve of the planet, till it soared into the daylight of Delta Andromedae.
Even in a high-class place that supposedly catered to all known physiologiesthe soothing, programmed play of lights across the columned walls, the shifting spectra that were supposed to relax weary travelers' central nervous systems, struck Zuckuss as crepuscular and depressing as the faded hopes of his youth.
Without dinosaurs dominating the ecosystem, mammals—or the endothermic octopeds on Beta Hydri III, and the live-birthers like T’kna on Delta Pavonis II—would never have been forced into the crepuscular existence that fostered the development of bigger brains.
Piemur and Farli were motionless in the crepuscular light that briefly heralded the dawn.