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An hour is a unit of time equal to 60 minutes, or 3,600 seconds. In civil timekeeping it is defined as one twenty-fourth of a mean solar day, which provides the basis for most daily scheduling worldwide. The abbreviation is h, and the plural form is hours. In everyday use, an hour refers to a roughly sixty-minute interval, although the exact length of clock time can be affected by local time standards and daylight saving practices.

The hour is a non-SI unit that is accepted for use with the International System of Units. It is defined exactly as 3,600 seconds. Related units include the minute (60 seconds) and the day (24 hours). Daylight saving time shifts clocks forward or backward by one hour in many regions, changing civil time labels but not the underlying physical length of an hour.

Historically, the concept arose from early timekeeping methods. Ancient cultures divided the day into hours using sundials or water clocks. In medieval Europe, daylight and night were each divided into 12 hours, so the length of an hour varied with the season. The modern fixed-length hour emerged with mechanical clocks and standardized time systems from the Early Modern period onward.

Today, hours are used to measure durations, schedule events, and express rates in science, transportation, and daily life. They can be stated as elapsed time (two hours), a time of day (at 07:00 hours), or in scientific calculations such as 3,600 seconds per hour.