If something neutralizes the effect of something else, then you can say the effect is negated. Hanging a disco ball from your living room ceiling negates the sleek modern effect created by the contemporary furniture.
If something is proved false or untrue, it has been negated. The discovery of one dinosaur jaw negated the conventional wisdom that all dinosaurs were vegetarians, since the tooth structure proved that guy definitely ate meat. The verb negate can also mean to counteract or counterbalance — so a really strong serve can negate your other weaknesses on the tennis court.