Reactive armor or explosive reactive armour (ERA), is a type of armour used primarily on tanks to lessen the damage from explosions caused from missile warheads, exploding shellss, grenades, or launched bombs.
Reactive armor's protective mechanism involves producing an explosion or other such reaction when it is impacted by a weapon, "pushing back" against it. This is particularly effective against shaped charge warheads, in which the warhead directs a focused jet of molten metal against the armor; reactive armor's reaction disrupts the jet before it reaches the armor's surface.
Modern ERA such as the Russian Kontakt-5 is made up of "bricks" of explosive sandwiched between two metal plates. The plates are arranged in such a way as to move sideways rapidly when the explosive detonates. This will force an incoming KE-penetrator or shaped charge jet to cut through more armour than the thickness of the plating itself, since "new" plating is constantly fed into the penetrating body. A KE-penerator will also be subjected to powerful sideways forces, which might be large enough to cut the rod into two or more pieces. This will significantly reduce the penetrating capabilities of the penetrator, since the penetrating force will be dissipated over a larger volume of armour.
ERA bricks are used as add-on armour to the most vulnerable parts of an armoured vehicle or tank. They require fairly heavy armour on the vehicle itself, since the exploding ERA would otherwise damage the vehicle and injure or kill the personnel inside.
Recent research has produced the idea of Electric Reactive Armour, where the armour is made up of two electrically charged plates separated by an insulator. When an incoming body penetrates the two plates and closes the circuit, a high voltage jolt will supposedly vaporize the penetrator and significantly reduce the resulting penetration. It is not public knowledge whether this is supposed to function against both KE-penetrators and shaped charges, or shaped charges alone. This technology has yet to be introduced on any operational platform.
Compare Chobham armor aka Composite armour.