Opposed check

From NWNWiki

An opposed check occurs when the success of an attack (spell, feat or otherwise) is determined by the target's skills, abilities or other attributes. This is different than a check against a set DC (as with disabling preset traps) or avoiding harmful effects (as with a saving throw).

An example of an opposed check is the Discipline check when attempting a Knockdown: if the attacker hits, the attack roll (attack bonus + d20 + size bonus) is opposed by the target's Discipline check (Discipline + d20), and the attacker must beat it (equal or surpass it) to succeed in knocking the target down.