Origins
The word 'harvest' has undergone a significant semantic narrowing over its long history in English. In Old English, 'hærfest' was simply the word for autumn — the third season of the year, between summer and winter. It is cognate with German 'Herbst' (which still means 'autumn' as its primary sense), Dutch 'herfst' (autumn), and Old Norse 'haust' (autumn). All derive from Proto-Germanic *harbistaz, from PIE *kerp- (to pluck, to gather, to harvest).
The PIE root *kerp- is remarkably well attested across the Indo-European family. Latin 'carpere' (to pluck, to seize, to enjoy) comes from the same root and produced English 'excerpt' (from 'excerpere,' to pluck out), 'scarce' (from Vulgar Latin *excarpus, plucked out, hence rare), and possibly 'carpet' (though this etymology is debated). Greek 'karpos' (καρπός, fruit — the thing that is plucked) produced 'pericarp' (the wall of a ripened ovary), 'endocarp,' 'mesocarp,' and the prefix 'carpo-' in botanical terminology. The semantic thread connecting all these words is the act of plucking or gathering — the fundamental agricultural gesture.
In Old English and Middle English, 'harvest' (hærfest, harvest) served as the standard name for the season we now call autumn. The association between the season and crop-gathering was inherent — autumn was 'harvest' because it was the time when you harvested — but the word's primary meaning was temporal (a season) rather than agricultural (an activity).
Latin Roots
The displacement of 'harvest' as the seasonal term occurred in the sixteenth and seventeenth centuries through the introduction of two competing words. 'Autumn' entered English from French 'automne' (from Latin 'autumnus') in the fourteenth century and gradually spread from literary and learned usage to general speech. 'Fall' arose as a shortening of the poetic phrase 'fall of the leaf' in the sixteenth century. By the seventeenth century, both 'autumn' (preferred in Britain) and 'fall' (which became dominant in North America) had displaced 'harvest' as the name of the season, and 'harvest' narrowed to its modern sense of 'the gathering of crops' or 'the yield of a season.'
German 'Herbst' provides a glimpse of what English 'harvest' would mean if it had not been displaced: 'Herbst' is simply the standard German word for autumn, with the agricultural sense secondary. Dutch 'herfst' follows the same pattern. English is the only major Germanic language in which the native word for autumn lost its seasonal meaning.
The word 'harvest' has rich metaphorical extensions: 'to harvest' organs (from the 1960s), 'to harvest' data (from the computing age), 'to reap what one has sown' (using 'reap,' the activity of harvest, as a moral metaphor). 'Harvest moon' (the full moon nearest the autumnal equinox, whose early rising allows farmers to work late into the evening) preserves the connection between the word and the agricultural calendar.