the season between summer and winter, when leaves change colour and the weather becomes cooler.——Longman Dictionary

The season between summer and winter (traditionally regarded as beginning with the annual harvest, and now generally regarded as lasting from September to November in the northern hemisphere and from March to May in the southern hemisphere), during which the leaves of many deciduous trees change colour and are shed, the nights get longer, and the weather typically becomes cooler.——Oxford English Dictionary  @en