The interval scale is a type of measurement scale where the interval between successive points can be measured using a defined numerical scale, however the zero point does not indicate absence of the characteristic being measured, but is arbitrary or undefined. Calendar time and temperature, in degrees centigrade, are examples of interval scales.
Examples of interval scales are fairly rare, but it is important to recognize them because some mathematical operations, including multiplication and division, are not valid for interval scales.
A scale that has the characteristics of an interval scale, but also zero does indicate the absence of the characteristic being measured (eg. length, weight) is known as a Ratio Scale.