Close bosom-friend of the maturing sun;
Conspiring with him how to load and bless
With fruit the vines that round the thatch-eves run;
To bend with apples the moss’d cottage-trees,
And fill all fruit with ripeness to the core;
To swell the gourd, and plump the hazel shells
With a sweet kernel; to set budding more,
And still more, later flowers for the bees,
Until they think warm days will never cease,
For Summer has o’er-brimm’d their clammy cells.
Keats To Autumn ll. 1-11
1. The season of the year between summer and winter, lasting from the autumnal equinox to the winter solstice and from September to December in the Northern Hemisphere; fall.
2. A period of maturity verging on decline (AHD).
Modern English autumn via Middle English autumpne, from Old French autompne, from Latin autumnus.
The etymology offered for autumn by the AHD seems clear enough, but the earlier history of autumn is not at all clear, once we track back to Latin autumnuns. The OED refers the etymologically curious to the standard Lewis and Short Latin dictionary. Lewis and Short suggests that Latin autumnus may be related to the older Latin augere, or "increase."
Standard English usage before about the sixteenth century favored harvest was the preferred name for this time of year; now, in North America, fall seems to be the more commonly used word. In any case, today while the sun is bright and the temperature moderate, the breeze sending leaves waft and skirling along the sidewalks is very much the signature of fall or autumn, and a harbinger of harvest to come.