the time of the year when the sun is hottest and the days are longest, between spring and autumn.——Longman Dictionary

The season between spring and autumn (now generally regarded as lasting from June to August in the northern hemisphere and from December to February in the southern hemisphere), during which the weather is typically warm or hot, the days are long, many animals are more active, and crops ripen, and which is popularly associated with school holidays and outdoor leisure activities. ——Oxford English Dictionary  @en