A brief or shortened form of 'diurnal'; historically used to describe a daily prayer service or religious office said during the day.
From Latin 'diurnus' (daily), this abbreviated form appears in medieval ecclesiastical contexts. It was shortened for practical use in religious manuscripts and service books.
Medieval religious life was organized around specific prayer times throughout the day—'diurn' referred to the daytime services as opposed to nighttime ones like 'matins.' This word is virtually extinct today, replaced by more specific modern liturgical terms.
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