Counter
Counter is term for spells that have counterpart spell which dispels (cancels) it, and which it dispels. This means that both counter spells in a pair can not be active on the same unit at the same time. When a counter spell is cast on a unit which is under the effects of its counterpart, the existing spell effect will be removed and the newly cast spell will take effect.
Counter spells:
= Opposite spells
Additionally, there are spell pairs that do not dispel each other, but their effect are equal and opposite and in that sense they cancel each other.
Opposite spells:
Stone Skin ←→ 
Weakness
Fortune ←→ 
Misfortune
Mirth ←→ 
Sorrow