coun·ter·vail 
(koun
′t
ər-v
āl
, koun
t
ər-v
āl
′)
v. coun·ter·vailed, coun·ter·vail·ing, coun·ter·vails
v.tr.1. To act against with equal force; counteract.
2. To compensate for; offset.
v.intr. To act against an often detrimental influence or power.
[Middle English countrevaillen, from Old French contrevaloir, contrevail- : contre-, counter- + valoir, to be worth (from Latin valēre, to be strong; see wal- in the Appendix of Indo-European roots).]