One of the mysteries of the English language finally explained.
1The length of the day, either: (a) the time between sunrise and sunset (i.e. the duration of daylight), especially as it varies at different times of the year or at different latitudes; or (b) the time from one sunrise to the next (on earth currently 24 hours). Also as a count noun: a day of a specified length.
2Biology. The length of daily illumination which an organism receives; the length of light exposure which is most effective in stimulating a particular physiological process, such as reproduction, in a plant or other organism; = "photoperiod".
Designating clothes (especially dresses and skirts) that are shorter in length than evening clothes, and typically intended for use during the day.
Mid 16th century. From day + length.
day length/ˈdeɪ lɛŋ(k)θ/
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