countermand
Concise Oxford English Dictionary © 2008 Oxford University Press:
countermand /ˌkaʊntəˈmɑːnd/
▶verb revoke (an order).
■ declare (voting) invalid.
– origin ME: from OFr. contremander (v.), contremand (n.), from med. L. contramandare, from L. contra- ‘against’ + mandare ‘to order’.
'countermand' also found in these Oxford entries:

