im·pair /im-'per/ vt
1: to damage or make worse by or as if by diminishing
impair 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 impair ing a state's own obligations was entitled to less deference — Gerald Gunther see also contract clause
im·pair·ment n
Merriam-Webster’s Dictionary of Law. Merriam-Webster. 1996.