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The early Romans had two words for "twilight." "Crepusculum" was favored by Roman writers for the half-light of evening, just after the sun sets; it is a diminutive formation based on their word for "dusky," which is "creper." "Diluculum" was reserved for morning twilight, just before the sun rises - it is related to lucidus, meaning "bright." We didn't embrace either of these Latin nouns as substitutes for our Middle English "twilight," but we did form the adjective "crepuscular" in the 17th century. At first, it only meant "dim" or "indistinct," often used in a figurative sense. In the 1820s, we added its special zoological sense, describing animals that are most active at twilight.
First Known Use of crepuscular
Medical Definition of crepuscular
1: of, relating to, or resembling twilight <crepuscular depths of personality—William James>
2: active in the twilight <crepuscular animals>
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Medical Dictionary: Definition of "crepuscular"
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