Countermagic

Countermagic is the use of magic to weaken, undo or counter a magical effect.

Description
Countermagic can be used both when a magical effect is being made or when one is active and the mage wants to undo it. The mage should be aware of the effect and have at least one dot in all the spheres involved in it to use countermagic (one dot in Prime allows the mage to turn the spell back on the caster). The mage can also make a opposing effect instead of trying to stop the one that's beying thrown at him, should he know what the effect is going to be and a way to counter it with other sphere.

Reference
Mage: The Ascension Revised Edition p. 154, 155