A supergiant is a star which is brighter and bigger than giant stars of the same colour and same temperature. Only the most massive stars become supergiants after the red giant stage.
A star of which the mass and diameter can be 100 times greater than the Sun's. Because of their size, their density is low and they rotate very slowly. They consume an enormous amount of energy and their life is relatively short: a few hundred million years. In the MK classification they are class I. e.g.: Rigel type B8Ia.