[en] The term 'shock' is defined. Shock waves are characterised by finite mass flux and discontinuity by zero mass flux across the surface of discontinuity. Contact discontinuity and tangential discontinuity are also defined. Collisionless shocks in solar wind (plasma) are of three modes viz., whistler, magneto-acoustic and Alfven. In the mode, laminar and turbulent shocks exist. The instabilities in the shocks are mentioned. Mach number is defined. Other types of nonlinear shocks such as envelope shocks and low shock are defined and explained. Various types of discontinuities observed in solar wind (plasma) are discussed. Emission of energetic particles from the plasma are briefly discussed. (A.K.)