The expression methods of interval are: (a, b) (b > a), (open interval); (a, b)(b > a), (semi-open and semi-closed interval); [a, b] (b > a), (semi-open and semi-closed interval); [a, b] (b > a), (closed interval).

In mathematics, an interval usually refers to a set of real numbers: if X and Y are two numbers in a set, then any number between X and Y also belongs to the set.

For example, a set consisting of real numbers that conform to 0≤x≤ 1 is an interval, which includes 0, 1 and all real numbers between 0 and 1. Other examples are: real number set, negative real number set, etc.