chiefly Brit the season between summer and winter, in which crops and fruit are gathered and deciduous trees lose their leaves, in the N hemisphere the months of September, October, and November
in astronomy, the period in the N hemisphere extending from the September equinox (time when day and night are of equal length) to the December solstice (shortest day of the year)
a period of maturity or the early stages of decline
autumnal /awʹtumnəl/ adj
[Middle English autumpne from Latin autumnus]