Aday (symbold) is aunit oftime equivalent to approximately 24hours. It is not anSI unit but it is accepted for use with SI. The SI unit of time is thesecond.
The word 'day' can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Both refer to a length of time. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of theweek or to acalendar date, as in answer to the question "On which day?".
The term comes from theOld Englishdæg, with similar terms common in all otherIndo-European languages, such asTag inGerman,dies inLatin,dydd inWelsh ordive inSanskrit.
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See also:Photographs taken on day 2021-01-01 (and the following days)