countervail \kown-tur-VAYL\ (transitive verb) - 1 : To act against with equal force, power, or effect; to counteract. 2 : To compensate for; to offset; to furnish or serve as an equivalent to.
(intransitive verb) - To exert force against an opposing, often bad, influence or power.
"In spite of its keel's weight, and even without the countervailing underwater resistance of its mast, Dubois's boat seemed comfortably stable upside down." -- Derek Lundy, 'Godforsaken Sea'
Countervail derives from Old French contrevaloir , from contre- , "counter-" (from Latin contra , "against") + valoir , "to be worth" (from Latin valere , "to be strong, to avail").