1580s, "an account of daily events, a journal kept by one person of his or her experiences and observations," from Latin diarium "daily allowance," later "a journal," neuter of diarius "daily," from dies "day" (from PIE root *dyeu- "to shine," in derivatives "sky, heaven, god"); also see -ary.
Sense of "a book with blank leaves or dated pages meant for keeping a daily record of events" is from c. 1600. Related: Diarial; diarian.
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