Webster's English Dictionary

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invalidate

Cross references:
  1. nullify               
 
in.val.i.date \(')in-'val-*-.da-t\ \(.)in-.val-*-'da--sh*n\ 
   \in-'val-*-.da-t-*r\ vt : to make invalid; esp : to weaken or destroy the 
   cogency of  - in.val.i.da.tion n