What is M20 concrete grade?

M20 refers to the grade of the concrete generally used in construction purpose. Here, M refers to mix & number 20 refers to the characteristic strength of concrete. the proportions of this grade is (1:1 1/2: 3). Along with this there are some more grades used in constructions i.e., M5, M7.5, M10, M15, M25, M30................. M80