Old English hærfest "autumn," as one of the four seasons, "period between August and November," from Proto-Germanic *harbitas (source also of Old Saxon hervist, Old Frisian and Dutch herfst, German Herbst "autumn," Old Norse haust "harvest"), from PIE root *kerp- "to gather, pluck, harvest."
In Old English and Middle English it was primarily a season name, with only an implied reference to the gathering of crops. The meaning "the time of gathering crops" is attested by mid-13c., and the sense was extended to the action itself and the product of the action (after c. 1300). After c. 1500 these were the main senses and the borrowed autumn and repurposed fall (n.) supplied the season name.
The figurative uses begin by 1530s. As an adjective, from late 14c. Harvest home (1570s) was a festive celebration of the bringing home the last of the harvest; harvest moon (1704) is that which is full within a fortnight of the autumnal equinox.
c. 1400, from harvest (n.). Of wild animals, by 1946; of cells, from 1946. Related: Harvested; harvesting.