Counter
Counter is term for spells that have counterpart spell, that together form a pair of spells that dispels (cancels) each others. This means, that counter spells can not be active on the same unit at the same time. When a counter spell is cast on a unit that has its counterpart active, the old spell effect will be canceled and the lastly cast spell will take effect.
List of true counter spells:
List of spells with opposite effects: