Autumn or Fall is regarded as the third season of the year, from the descending or autumnal equinox to the winter solstice, approximately September 21 to December 21. Chaucer first used the word autumn c 1374, which is derived from Latin autumnus/auctumnus. The use of fall to mean autumn in North American English comes from the phrase fall of the leaf and it came into use by 1545 for this time of year when the leaves fall from the trees. The term autumn is still preferred in British English.

An equinox, literally 'equal night' from Latin aequinoctium, occurs twice a year when the Sun crosses the equator and day and night are equal in length. The autumnal equinox is actually the moment at which the Sun crosses the equator, usually on September 22 or 23.

The word foliage first appeared in late Middle English, was spelled foilage, and meant 'a design resembling leaves' though quite quickly it took on the collective meaning of 'leaves of a plant or tree.' The word traces back through French to Latin folium, 'leaf,' which influenced the later spelling change. In the fall, there are many foliage tours and leaf-peeper trips to see the spectacular changes in leaf colors.

Leaves are green because of pigments called chlorophylls. When chlorophylls are abundant in the leaves' cells during the growing season, the chlorophylls' green color dominates and masks the colors of any other pigments in the leaves. Thus, the leaves of summer are characteristically green. But as autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used. The "masking" effect slowly fades and the other pigments that have been present begin to show. These are called carotenoids and they give the leaves colors of yellow, brown, and orange. The reds, the purples, and their blended combinations come from another group of pigments in the cells called anthocyanins. These pigments are not present in the leaves throughout the growing season but develop in late summer in the sap of the leaf cells due to complex interactions both inside and outside the plant. The formation of anthocyanins depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaves is reduced.

Harvest is from an Old English word haerfest which meant 'autumn' which had Germanic origins and shared an Indo-European root with related forms like Latin carpere and Greek karpos. Harvest's original meaning in English was 'autumn' and then came to refer to the season for reaping and gathering grain and other grown products. The full moon nearest the autumnal equinox is called the Harvest Moon. Near the time of the autumnal equinox, the angle of the moon's orbit relative to the Earth's horizon is at its minimum, causing the full moon to rise above the horizon much faster than usual. Because the harvest moon, like any full moon, must rise near the hour of sunset, harvest workers in the Northern Hemisphere may be aided by bright moonlight after sunset.

A common autumn phenomenon in the central, eastern, and northern United States and in Europe is Indian summer, a period of unseasonably warm and dry weather that sometimes occurs in late October or November. The name may come from this phenomenon having first been noticed in the region occupied by the Native Americans (Indians) in North America.

The names of the months September, October, November are rooted in Latin. September is from septem 'seventh month' of the early Roman calendar - though it is now the ninth month in the Gregorian calendar. October (octo) is Latin for 'eighth month' (now tenth) and November (novem) is Latin for 'ninth month' (now eleventh in the Gregorian calendar). In Old English, September was called 'harvest month.'