Nullification is the act of cancelling something. Counteracting the effects of a snakebite with an antidote could be described as nullification, for example.
Use the noun nullification when one thing overcomes or overrides another, basically erasing the effects of the first thing. Nullification of a newly passed law would occur if the law turned out to be impossible to enforce. The word is most often used in a political sense, in fact, and was first used this way by Thomas Jefferson in 1798. The Latin root is nullificationem, "a making as nothing," from nullus, "not any."