A reprisal is an act of retaliation, especially one committed by one country against another. If you attack your enemy's village and cause lots of damage, expect a reprisal.
Reprisal comes from the French for taking back, and used to mean the seizure of property as a compensation for some earlier loss. Now we use it more in the sense of a retaliatory attack. When Germany bombed London during World War II, the British reprisals included the bombing of Berlin. Reprisal doesn't always have to be about war; you can use it for any act of retaliation.