WordNet (r) 3.0 (2006):

mental deficiency
    n 1: mild mental retardation [syn: moronity, mental
         deficiency]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "mental deficiency":
   aberration, abnormality, alienation, amentia, arrested development,
   backwardness, blithering idiocy, brain damage, brainsickness,
   clouded mind, craziness, cretinism, daftness, dementedness,
   dementia, derangement, disorientation, distraction, folie, furor,
   half-wittedness, idiocy, idiotism, imbecility, infantilism,
   insaneness, insanity, irrationality, loss of mind, loss of reason,
   lunacy, madness, mania, mental defectiveness, mental derangement,
   mental disease, mental disorder, mental disturbance,
   mental handicap, mental illness, mental instability,
   mental retardation, mental sickness, mind overthrown, mindsickness,
   mongolianism, mongolism, mongoloid idiocy, moronism, moronity,
   oddness, pixilation, possession, profound idiocy, queerness,
   rabidness, reasonlessness, retardation, retardment, senselessness,
   shattered mind, sick mind, sickness, simple-wittedness,
   simplemindedness, simpleness, simplicity, strangeness,
   subnormality, unbalance, unbalanced mind, unsaneness, unsound mind,
   unsoundness, unsoundness of mind, witlessness