Counter Hit (カウンターヒット Kauntā Hitto), also known simply as Counter, is the term that describes the effect of attacking an opponent while they try to attack the player. Supercharger can also be used to give a hit the properties of a Counter Hit, however, damage is calculated differently and will take precedence over a legitimate Counter Hit.
Counter Hits inflict more damage and sometimes have different effects. An example of the former is Armor King's "Low Drop Kick", which will do 28 (116%) damage on a Counter Hit (it will usually just cause 24 (100%) damage.).
An example of the latter is King's "Konvict Kick". If it is a Counter Hit, King will perform a "DDT" after the kick. Other effects can includes stuns, knocking the opponent to the ground, or launching the opponent. Counter hits can also be used to avoid unblockable attacks.