Counterspell is a card played in Magic: The Gathering that stops spells cast by other players and prevents their potential effects. As GatheringMagic.com points out, counter magic has been considered too powerful at times in the history of the game because it was stronger than the spells being cast.
Counterspell is most useful and least disruptive to the game when a player uses it to keep himself from falling behind in the game. When it is used to allow a player to pull ahead, it can be oppressive to gameplay. Counter magic is most effective when it is used to keep powerful spells in check.