A closed interval is a range of numbers that includes both its endpoints. It is typically represented in mathematical notation as [a, b], where a and b are the lower and upper bounds, respectively. This means that all numbers between a and b, including a and b themselves, are part of the interval. Closed intervals are commonly used in mathematics, particularly in calculus and analysis, to define sets of real numbers. They help in understanding concepts like continuity and limits. For example, the closed interval [1, 5] includes all numbers from 1 to 5, such as 1, 2, 3, 4, and 5.