Debuff

As a noun, a debuff is any ability that reduces an attribute of an opponent. However, there are no skills in Guild Wars that actually reduce attributes, rendering this term a bit of a misnomer; usually, if something is referred to as a debuff, it causes a condition on the opponent, which then reduces the effectiveness of the afflicted. This term is also used to reference an ability that causes health or energy degeneration.

As a verb, "to debuff" means to use a debuff on a target.