term:

Fatal

fatal adj

1 : causing failure of a legal claim or cause of action [a defect in the proceedings "W. R. LaFave and J. H. Israel"]
2 : making something (as a contract) invalid or unenforceable [there is a indefiniteness with the result that the agreement is void "J. D. Calamari and J. M. Perillo"]
fa·tal·ly adv

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.