Traditionally the third of the four seasons, when deciduous trees lose their leaves; typically regarded as being from September 24 to December 22 in parts of the Northern Hemisphere, and the months of March, April and May in the Southern Hemisphere.
"As autumn arrives in the Northern Hemisphere during late September through November, the maple trees begin to shed their vibrant foliage."
In plain English: Autumn is the season after summer when leaves change color and fall from trees before winter starts.
"The leaves turned bright colors and fell from the trees this autumn."
Usage: Use "autumn" primarily for the season itself rather than describing something related to it. When referring to characteristics of this time, such as colors or moods, switch to the adjective form "autumnal."
Of or relating to autumn; autumnal
"The crisp air carried the distinct scent of fallen leaves, marking the true arrival of autumn."
"The autumn leaves turned vibrant shades of red and orange before falling to the ground."
A female given name from English of modern usage, from the name of the season.
"Autumn was thrilled to finally have a baby girl named after her favorite season."
The word "autumn" comes from the Latin autumnus. It was borrowed through Old French and Middle French into Middle English.