Any one of the periods, longer or shorter, into which the year is naturally divided by the earth's changing position in regard to the sun, and which are marked by varying length of day and night, by particular conditions of weather, temperature, etc. More specifically, each of the four equal periods—Spring, Summer, Autumn, Winter—into which the year is divided by the passage of the sun from equinox to solstice and from solstice to equinox; also, each of the two periods—the rainy and the dry—into which the year is divided in tropical climates.——牛津当代大辞典

one of the main periods into which a year is divided, each of which has a particular type of weather. The seasons are spring, summer, autumn, and winter.——朗文当代大辞典  @en