[Spells help] counterspell

A counterspell is a term used to describe two spells which have opposite
effects and therefore have the ability to cancel each other.  For example,
if a successful slow is cast on a hasted target, the haste spell would be
negated INSTEAD of the slow spell taking effect.  The caster would need to
cast slow a second time in order to actually produce a slow effect.

See also: slow weaken