An hour is a unit of time conventionally reckoned as ​1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.

The hour was initially established in the ancient Near East as a variable measure of ​1⁄12 of the night or daytime. Such seasonal, temporal, or unequal hours varied by season and latitude. The hour was subsequently divided into 60 minutes, each of 60 seconds. Equal or equinoctial hours were taken as ​1⁄24 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it ​1⁄24 of the mean solar day. Since this unit was not constant due to long term variations in the Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of the atomic or physical second.

In the modern metric system, hours are an accepted unit of time defined as 3,600 atomic seconds.

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