A set of real numbers bounded by two real numbers - the endpoints or bounds. The set may or may not include either endpoint, leading to four possibilities:

 closed                   [a, b]  a <= x <= b
 open                     (a, b)  a < x < b
 left-open, right-closed  (a, b]  a < x <=b
 left-closed, right-open  [a, b)  a <= x < b

See closed interval, open interval.