A time period of sixty minutes; one twenty-fourth of a day.
"The meeting was scheduled for exactly one hour, so we need to leave promptly at six o'clock."
In plain English: An hour is a unit of time that lasts for sixty minutes.
"The meeting will last for exactly one hour."
Usage: Use "hour" to refer to a standard sixty-minute block of time or a specific point on the clock, such as meeting at three o'clock. Do not confuse it with "hours" when referring to multiple periods or durations, like studying for two hours.
The word "hour" entered English from the Old French heure, which was borrowed from Latin hōra. This term originally referred to any period of time or season and is related to the Proto-Indo-European root for "year."