1. One of the 24 equal parts of a day: It took Janet only one hour to do her homework for school for the next day.
2. One of the points on a timepiece marking off in 12 or 24 successive intervals of 60 minutes, from midnight to noon and noon to midnight or from midnight to midnight: The clock stuck two times when it reached the hour or number 2 on the face of the clock.
3. The time of day as indicated by a 12-hour clock and the time of day determined on a 24-hour basis: 1730 hours is 5:30 p.m.
4. A time emphasised to mean that something lasts or takes a long time (only pl): It took hours and hours to get to L.A. because of so many construction sites on the freeway.
5. A customary or fixed time: The hour for dinner time at the Smith's home was at 6 o'clock in the evening.
6. Etymology: from Old French hore, "one-twelfth of a day" (sunrise to sunset), from Latin hora, "hour, time, season"; from Greek hora. "any limited time"; referring to a day, hour, season, and year.

The Greeks borrowed the notion of dividing the day into hours from the Babylonians, but the Babylonian hour was one-twelfth of the whole day and therefore twice as long as a modern hour. The Greeks divided only the period of light into 12 parts, and the Romans adopted the system from them.

Night was not similarly divided until much later, and so the period of time covered by an hour varied according to the season.

In the 16th century, a distinction was sometimes made between temporary (unequal) hours and sidereal (equal) ones.