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coined 1959 from Latin circa "about" (see circa) + diem, accusative singular of dies "day" (see diurnal). The original use is in circadian rhythm.
circadian cir·ca·di·an (sər-kā'dē-ən, -kād'ē-, sûr'kə-dī'ən, -dē'-)adj. Relating to biological variations or rhythms with a cycle of about 24 hours.