Hour/ˈaʊər/ Definition 1 of 4Pro Tip 1/3Duration Reference Use 'hour' when indicating the length of time something takes. The test will last an hour. Pro Tip 2/3Approximation 'About an hour' implies a rough estimate, not exactly 60 minutes. I'll be back in about an hour. Pro Tip 3/3Common Phrases 'On the hour' means when the clock strikes a full hour. The show starts on the hour.