Reactive armor actively reduces the damage a vehicle equipped with it receives, instead of passively resisting it like traditional armor.
The common type of reactive armor currently is explosive reactive armor (ERA), which will detonate when penetrated and impair or deflect the incoming projectile. This type of armor appears as tiles or plates fitted on top of the normal armor of the vehicle equipped with it.
Once their protective surface is spent they can be removed to save weight.
This armored defense system has also been featured in several modern mecha anime like Gundam and MACROSS.