an·nul
 /əˈnʌl/  verb  , -nuls, -nulled, -nul·ling [  + obj  ]
  : to say officially that something is no longer valid : to make (something) legally void
   Their marriage was annulled.
   annul a contract/vote/election
  an·nul·ment /əˈnʌlmənt/  noun  ,  pl   -ments [  count  ]
   They wanted an annulment of their marriage.
  [  noncount  ]
   The couple has filed for annulment.