In a planetary magnetosphere, the bow shock is the boundary at which the solar wind abruptly drops as a result of its approach to the magnetopause. The defining criterion is that the bulk velocity drops from "supersonic" to "subsonic", where these terms have meanings that are specialized to plasma physics. The particles making up the solar wind follow spiral paths along magnetic field lines. The velocity of each particle as it gyrates around a field line can be treated similarly to a thermal velocity in an ordinary gas, and in an ordinary gas, the mean thermal velocity is roughly the speed of sound. At the bow shock, the bulk forward velocity of the wind (which can be seen as the velocy of the points on the field lines about which the particles gyrate) drops below the speed at which the particles are corkscrewing.
The Sun also has a bow shock as it travels through the interstellar medium.
