late 14c., autumpne (modern form from 16c.), from Old French autumpne, automne (13c.), from Latin autumnus (also auctumnus, perhaps influenced by auctus "increase"), which is of unknown origin. Perhaps from Etruscan, but Tucker suggests a meaning "drying-up season" and a root in *auq- (which would suggest the form in -c- was the original) and compares archaic English sere-month "August." De Vaan writes, "Although 'summer', 'winter' and 'spring' are inherited IE words in Latin, a foreign origin of autumnus is conceivable, since we cannot reconstruct a PIE word for 'autumn'".
Harvest (n.) was the English name for the season until autumn began to displace it 16c. Astronomically, from the descending equinox to the winter solstice; in Britain, the season is popularly August through October; in U.S., September through November. Compare Italian autunno, Spanish otoño, Portuguese outono, all from the Latin word.
As de Vaan notes, autumn's names across the Indo-European languages leave no evidence that there ever was a common word for it. Many "autumn" words mean "end, end of summer," or "harvest." Compare Greek phthinoporon "waning of summer;" Lithuanian ruduo "autumn," from rudas "reddish," in reference to leaves; Old Irish fogamar, literally "under-winter."