Restrictive cardiomyopathy (RCM) is a form of cardiomyopathy in which the walls of the heart are rigid (but not thickened). Thus the heart is restricted from stretching and filling with blood properly. It is the least common of the three original subtypes of cardiomyopathy: hypertrophic, dilated, and restrictive. It should not be confused with constrictive pericarditis, a disease which presents similarly but is very different in treatment and prognosis.

Reference
Wiki: restrictive cardiomyopathy