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1. Declare invalid
   "The contract was nullified";
   - invalidate, annul, quash, void, avoid
    
2. Show to be invalid
   - invalidate
    
3. Make ineffective by counterbalancing the effect of
   "Her optimism nullifies his gloom";
   - neutralize, neutralise [Brit], negate

Derived forms: nullified, nullifies, nullifying

Type of: cancel, contradict, negate, strike down, weaken

Antonym: validate

Encyclopedia: Nullify