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A day (symbol d) is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second.

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 the week or to a calendar date, as in answer to the question "On which day?".

The term comes from the Old English dæg, with similar terms common in all other Indo-European languages, such as Tag in German, dies in Latin, dydd in Welsh or dive in Sanskrit.