According to the American Heritage Dictionary
- A non-spatial continuum in which events occur in apparently irreversible succession from past through the present to the future.
- One of the 24 equal parts of a day.
- One of the points on a timepiece marking off 12 or 24 successive intervals of 60 minutes, from midnight to noon and noon to midnight or from midnight to midnight.
For ancient civilizations, the hour was one of the twelve parts in which time was divided, between the sunrise and sunset, later on also dividing the night in other twelve hours, giving origin to the day of twenty-four hours that we know nowadays. The Egyptians were probably the first ones to divide the day into smaller units