mitral stenosis: obstruction or narrowing of the mitral valve (as by scarring from rheumatic fever)
valvular heart disease: heart disease caused by stenosis of the cardiac valves and obstructed blood flow or caused by degeneration and blood regurgitation
antiparticle: a particle that has the same mass as another particle but has opposite values for its other properties; interaction of a particle and its antiparticle results in annihilation and the production of radiant energy