(units of measure) A unit of time reckoned as ¹⁄₁₂ of a day's daylight or nighttime, varying according to season, latitude, and local weather.

The number of credit hours a student takes in a semester, usually for a college or university to count towards a degree.

The scheduled time for the start of some event, especially a military operation; H-hour.

Just before it is too late; in the nick of time; last-minute.

(time) fifteen minutes.

The hour after midnight, when witches and other supernatural beings were thought to be active, and to which bad luck was ascribed.

The times, typically from about 9am to 5pm, Monday to Friday, when non-24/7 office workers are at their desks.

The regular times of day when a shop, office or similar is open to the public.

(electricity) A unit of energy equal to that done by one kilowatt acting for one hour.

(idiomatic) A point in time which is nearly too late; the last minute.