Annus (“year”) + Versum (“Having been turned”)"year-day, annual return of a certain date in the year," originally especially of the day of a person's death or a saint's martyrdom, from Medieval Latin anniversarium, noun from Latin anniversarius (adj.) "returning annually," from annus (genitive anni) "year" + versus, past participle of vertere "to turn."late 14c., "appointed by the year;" c. 1400, "occurring or done once a year," from Old French annuel "yearly" (12c.) or directly from Medieval Latin annualis "yearly," corresponding to Latin annalis as adjective form of annus "year."This is reconstructed to be from Proto-Italic *atno- "year" (compare Oscan akno- "year, holiday, time of offering"), from PIE *at-no- "which goes," also "a year" (as "going around"), suffixed form of root *at- "to go" (source also of Sanskrit atati "goes, wanders," atamana- "to travel, wander," atya- "steed, runner"). The root also has Germanic derivatives meaning "a year," such as Gothic aþnam (dative plural) "year.""in the year of the Christian era," 1570s, Latin, literally "in the year of (our) Lord," from ablative of annus "year" + Late Latin Domini, genitive of Dominus "the Lord"Medieval Latin, "in the year of the hegira," the flight of Muhammad from Mecca, 622 C.E., from which Muslims reckon time; from ablative of annus "year" (see annual (adj.)) + genitive of hegira. Abbreviated A.H."consisting of or lasting 100 years, happening every 100 years," 1789, from Latin centum "one hundred" + ending from biennial. As a noun, "a hundredth anniversary celebration," 1620s, "lasting for two years;" 1750, "occurring every two years," from Latin biennium "two-year period," from bi- "two" + annus "year." 
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