hour

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of the 24 equal parts of a day.
• noun 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.
• noun The time of day indicated by a 12-hour clock.
• noun The time of day determined on a 24-hour basis.
• noun A unit of measure of longitude or right ascension, equal to 15° or ¹/₂₄ of a great circle.
• noun A customary or fixed time.
• noun A set or customary period of time for a specified activity.
• noun A particular time.
• noun A significant time.
• noun The present time.
• noun The work that can be accomplished in an hour.
• noun The distance that can be traveled in an hour.
• noun A single session of a school day or class.
• noun A credit hour.
• noun Ecclesiastical The canonical hours.
• idiom (long hours) A longer than usual or customary period of time for a given activity.

from The Century Dictionary.

• noun A particular time; a fixed or appointed time; a set season: as, the hour of death.
• noun The time marked or indicated by a timepiece; the particular time of day: as, what is the hour? at what hour shall we meet?
• noun The twenty-fourth part of a civil day, or the twelfth part of a natural day or night.
• noun plural Set times of prayer; the canonical hours (which see, under canonical).
• noun The offices or services prescribed for the canonical hours, or a book containing them. See book of hours, below.
• noun In Greek myth, one of the Horæ or Hours, the goddesses of the seasons and guardians of the gates of heaven.
• noun The hour reckoned from sunrise as the beginning of the day.
• noun In astronomy and geography, an angular measure of right ascension or longitude, being the twenty-fourth part of a great circle of the sphere, or fifteen degrees.
• noun One hour in a shop. In many technical schools students are required to spend a certain number of hours in workshops. These are called shop-hours, to distinguish them from the hours spent in the recitation-room.

from the GNU version of the Collaborative International Dictionary of English.

• noun The twenty-fourth part of a day; sixty minutes.
• noun The time of the day, as expressed in hours and minutes, and indicated by a timepiece; as, what is the hour? At what hour shall we meet?
• noun Fixed or appointed time; conjuncture; a particular time or occasion
• noun (R. C. Ch.) Certain prayers to be repeated at stated times of the day, as matins and vespers.
• noun A measure of distance traveled.
• noun after the time appointed for one's regular labor.
• noun See under Canonical.
• noun (Astron.) the angle between the hour circle passing through a given body, and the meridian of a place.
• noun (Astron.) A small brass circle attached to the north pole of an artificial globe, and divided into twenty-four parts or hours. It is used to mark differences of time in working problems on the globe.
• noun the hand or index which shows the hour on a timepiece.
• noun (Astron.), (Dialing) A line on which the shadow falls at a given hour; the intersection of an hour circle which the face of the dial.
• noun the plate of a timepiece on which the hours are marked; the dial.
• noun the twenty-fourth part of a sidereal day.
• noun the twenty-fourth part of a solar day.
• noun the early hours of the morning, as one o'clock, two o'clock, etc.
• noun to be regular in going to bed early.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A time period of sixty minutes; one twenty-fourth of a day.
• noun A season, moment, time or stound.
• noun poetic The time.
• noun military, in the plural Used after a two-digit hour and a two-digit minute to indicate time.