Counter-Bombardment is a rare ability to return long-range fire. If the unit in question is attacked by <l:6,19>Bombardment<e> by the enemy, it will simultaneosly retaliate with its own <l:6,19>Bombardment<e> attack. Still, the unit must be able to normally bombard that type of enemy.
Counter-Bombardment might cause collateral damage. If the attack was initiated from within a stack, any unit within the stack is in risk of being hit, unless immune to the Ranged Attack of the counter-bombarding unit.