Comment I went with IAU, but here's my definition anyway (Score 1) 454
Seasons are defined by the length of diurnal periods and the direction of the change:
Winter - Daylight period is increasing, lower than night period (lowest amount of daylight)
Spring - equal diurnal periods, daylight increasing
Summer - Daylight period is decreasing (greatest amount of daylight)
Autumn - equal periods, daylight decreasing