Crepuscular \kri-PUS-kyuh-lur\ , adjective;
1. Of, pertaining to, or resembling twilight; dim
2. (zoology) Appearing or active at twilight
This word perfectly conjurs up an image of twilight; it's dark and a little damp, you can hear the scratches and shuffles of nocturnal creatures greeting their new day. The rustle of leaves in the cool night air makes you shiver and the sound of something large moving in the bushes startles you. A far away coyote howls at the moon as you pull your jacket tight and briskly begin your walk home.
According to Etymonline.com:
This word was used in a figurative sense in the late 1600's and early 1700's, but the meaning became more literal around 1755. It derives from Latin crepusculum, meaning "twilight, dusk". Crepusculum comes from creper ("dusky") which is of unknown origin. The word generally refers to evening twilight, as opposed to dawn.
I'm not sure I understand the figurative v. literal sense of this word. I'm guessing the figurative sense was something like, "Why are you trying to read in this crepuscular room? It'll ruin your eyes." Since the room isn't affected by sunset like nature is, it's a figurative usage. The literal sense could really mean the zoological application, especially since the online etymology gives an exact year rather than a decade or century. If an existing word was coined as a scientific term it might be easier to pinpoint an origin, unlike spoken language, in which is almost impossible to determine exactly when and where words enter the vocabulary.
*Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Friday, October 22