In space physics, the termination shock is the boundary marking one of the outer limits of the sun's influence. It is where the bubble of solar wind particles slows down to subsonic speed (with respect to the star) due to interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. The termination shock is believed to be 75-90 astronomical units[1] from the Sun. The termination shock boundary fluctuates in its distance from the sun as a result of fluctuations in solar flare activity, i.e. changes in the ejections of gas and dust from the sun.

Going outward from the sun, the termination shock is followed by the Heliopause where solar wind particles are stopped by the interstellar medium, then the Bow Shock past which particles from the interstellar medium are no longer excited.