The Collaborative International Dictionary of English v.0.48:

Impairment \Im*pair"ment\, n. [OE. enpeirement, OF. empirement.]
   The state, act, or process of being impaired; injury. "The
   impairment of my health." --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impairment
    n 1: the occurrence of a change for the worse [syn: damage,
         harm, impairment]
    2: a symptom of reduced quality or strength [syn:
       deterioration, impairment]
    3: the condition of being unable to perform as a consequence of
       physical or mental unfitness; "reading disability"; "hearing
       impairment" [syn: disability, disablement, handicap,
       impairment]
    4: damage that results in a reduction of strength or quality
    5: the act of making something futile and useless (as by
       routine) [syn: stultification, constipation,
       impairment, deadening]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "impairment":
   adulteration, attrition, curtailment, cut, cutting, damage,
   debilitation, decrease, decrement, defectibility, defectiveness,
   deficiency, depletion, depreciation, derogation, deterioration,
   detraction, detriment, diminution, dip, disadvantage,
   disparagement, drawback, enfeeblement, erroneousness, extraction,
   fallibility, faultiness, flaw, handicap, harm, hurt, immaturity,
   imperfection, impurity, inaccuracy, inadequacy, inadequateness,
   incompleteness, inexactitude, inexactness, injury, lack, lessening,
   liability, loss, loss of ground, marring, mediocrity, mischief,
   patchiness, prejudice, reduction, remission, retraction,
   retrenchment, shortage, shortcoming, shortening, shrinkage,
   sketchiness, step backward, truncation, undermining, undevelopment,
   unevenness, unperfectedness, unsoundness, vitiation, want,
   weakening, worsening