To nullify something means to make it invalid or ineffective. A peace treaty is an attempt to nullify aggression and division within a region.

If you take a null, or a zero, and make it into an action you can take, and you’ve got nullify — the act of making something void or zero-like. If you have an argument with your friend, it might nullify the fun you had together that day. This word is particularly used in legal language — a divorce nullifies a marriage — or in business disputes where you are trying to nullify someone else’s actions or plans.

Definitions of nullify
  1. verb
    declare invalid
    synonyms: annihilate, annul, avoid, invalidate, quash, void
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    formalise, formalize, validate
    declare or make legally valid
    invalidate by judicial action
    printing: cancel, as of a correction or deletion
    type of:
    cancel, strike down
    declare null and void; make ineffective
  2. verb
    show to be invalid
    synonyms: invalidate
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    prove valid; show or confirm the validity of something
    type of:
    contradict, negate
    prove negative; show to be false
  3. verb
    make ineffective by counterbalancing the effect of
    synonyms: negate, neutralise, neutralize
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    type of:
    lessen the strength of
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