Hour is a development of the Anglo-Norman houre and Middle English ure, first attested in the 13th century.[b]
It displaced tide tīd, "time" and stound stund, span of time. The Anglo-Norman term was a borrowing of Old French ure, a variant of ore, which derived from Latin hōra and Greek hṓrā (ὥρα).
Like Old English tīd and stund, hṓrā was originally a vaguer word for any span of time, including seasons and years. Its Proto-Indo-European root has been reconstructed as *yeh₁- ("year, summer"), making hour distantly cognate with year.
The time of day is typically expressed in English in terms of hours. Whole hours on a 12-hour clock are expressed using the contracted phrase o'clock, from the older of clock. (10 am and 10 pm are both read as "ten o'clock".)
Hours on a 24-hour clock ("military time") are expressed as "hundred" or "hundred hours". (1000 is read "ten hundred" or "ten hundred hours"; 10 pm would be "twenty-two hundred".)
Fifteen and thirty minutes past the hour is expressed as "a quarter past" or "after" and "half past", respectively, from their fraction of the hour. Fifteen minutes before the hour may be expressed as "a quarter to", "of", "till", or "before" the hour. (9:45 may be read "nine forty-five" or "a quarter till ten".)