The hour (symbol: h)'s a unit of time. It isn't an SI unit but's accepted for use with the SI. Definition In modern usage, an hour's a unit of time 60 minutes, or 3,600 seconds in length. It's 1/24 of a median Earth day. An hour on the UTC time scale can include a negative or positive leap second, and can therefore also have a length of 3,599 or 3,601 seconds. Etymology Middle English ure first appears in the 13th century, as a loanword from Old French ure, ore, from Latin hora, ultimately from… (More on Hour)