In mathematics, an interval is a set of all real numbers lying between two fixed endpoints with no "gaps"). Each endpoint is either a real number or positive or negative infinity, indicating the interval extends without a bound. An interval can contain neither endpoint, either endpoint, or both endpoints. For example, the set of real numbers consisting of 0, 1, and all numbers in between is an interval, denoted) and called the unit interval; the set of all positive real numbers is an interval, denoted (0, ∞); the set of all real numbers is an interval, denoted (−∞, ∞); and any single real number a is an interval, denoted ).

Intervals can also be used to describe time intervals. By definition, a time interval is the amount of time between two given times. For example, the interval between 10:10 a.m. and 2:45 p.m. is 4 hours and 35 minutes.