millennium n.

"ten hundred thousand, a thousand thousands," late 14c., milioun, from Old French million (late 13c.), from Italian millione (now milione), literally "a great thousand," augmentative of mille "thousand," from Latin mille, which is of uncertain origin. From the start often used indefinitely for "a very great number or quantity."

In the West it was used mainly by mathematicians until 16c., but India, with its love of large numbers, had names before 3c. for numbers well beyond a billion. The ancient Greeks had no name for a number greater than ten thousand, the Romans for none higher than a hundred thousand. "A million" in Latin would have been decies centena milia, literally "ten hundred thousand." Million to one as a type of "long odds" is attested from 1761. Related: Millions.

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."