autumnal equinoxOur editors are putting together more complete content — check back in about 20 minutesautumnal equinoxautumnal equinox — noun名詞1. the time of year when day and night are equal in length, occurring in September1.名詞釋義the time of year when day and night are equal in length, occurring in September例句The autumnal equinox marks the beginning of autumn in the Northern Hemisphere.