Time Before The Modern Era

Divine Office -- The service of prayer and praise, the order of the day:

Matins -- the "night office", often joined to Lauds.
Lauds -- the "office of the dawn"
Prime -- a "little hour"
Terce -- "
Sext -- "
None -- "
Vespers -- the "evening hour", often sung between 3 and 6 pm
Compline -- said before going to bed

The day is divided by hours primarily, but these hours are not 60 minutes. An hour is just the time in one of these categories. So, matins, the "vigil" hour, is all night long, while Vespers is usually about 3 hours, and the "little hours" are less. So, the day is defined by the obligations of the Divine Office.