term:

Impair

impair vt

1 : to damage or make worse by or as if by diminishing [ed health]
2 : to diminish the value of (property or property rights)
;specif
: to diminish the value of (legal contractual obligations) to the point that a party loses the benefit of the contract or the contract otherwise becomes invalid [a law ing a state's own obligations was entitled to less deference "Gerald Gunther"] see also contract clause
im·pair·ment n

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