Interval -- from Wolfram MathWorld

An interval is a connected portion of the real line. If the endpoints a and b are finite and are included, the interval is called closed and is denoted [a,b]. If the endpoints are not included, the interval is called open and denoted (a,b). If one endpoint is included but not the other, the interval is denoted [a,b) or (a,b] and is called a half-closed (or half-open interval). An interval [a,a] is called a degenerate interval. If one of the endpoints is +/-infty, then the interval still...