[au.tumn] n [ME autumpne, fr. L autumnus] (14c) 1: the season between summer and winter comprising in the northern hemisphere usu. the months of September, October, and November or as reckoned astronomically extending from the September equinox to the December solstice--called also fall
2: a period of maturity or incipient decline "in the ~ of life" -- au.tum.nal adj -- au.tum.nal.ly adv