hour (n.) Look up hour at Dictionary.com
mid-13c., from Old French hore "one-twelfth of a day" (sunrise to sunset), from Latin hora "hour, time, season," from Greek hora "any limited time," from PIE *yor-a-, from root *yer- "year, season" (see year). Greek hora was "a season; 'the season;'" in classical times, sometimes, "a part of the day," such as morning, evening, noon, night.

The Greek astronomers apparently borrowed the notion of dividing the day into twelve parts (mentioned in Herodotus) from the Babylonians (night continued to be divided into four watches), but as the amount of daylight changed throughout the year, the hours were not fixed or of equal length. Equinoctal hours did not become established in Europe until the 4c., and as late as 16c. distinction sometimes was made between temporary (unequal) hours and sidereal (equal) ones. The h- has persisted in this word despite not being pronounced since Roman times. Replaced Old English tid, literally "time" (see tide (n.)) and stund "period of time, point of time, hour" (compare German Stunde "hour"), As a measure of distance ("the distance that can be covered in an hour") it is recorded from 1785.