The Collaborative International Dictionary of English v.0.48:

Derangement \De*range"ment\, n. [Cf. F. d['e]rangement.]
   The act of deranging or putting out of order, or the state of
   being deranged; disarrangement; disorder; confusion;
   especially, mental disorder; insanity.

   Syn: Disorder; confusion; embarrassment; irregularity;
        disturbance; insanity; lunacy; madness; delirium; mania.
        See Insanity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

derangement
    n 1: a state of mental disturbance and disorientation [syn:
         derangement, mental unsoundness, unbalance]
    2: the act of disturbing the mind or body; "his carelessness
       could have caused an ecological upset"; "she was unprepared
       for this sudden overthrow of their normal way of living"
       [syn: upset, derangement, overthrow]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "derangement":
   aberrance, aberrancy, aberration, abnormality, abnormity,
   alienation, amorphism, anomalism, anomalousness, anomaly,
   brain damage, brainsickness, clouded mind, confusion, convulsion,
   craziness, daftness, dementedness, dementia, deviation, difference,
   disarrangement, disarray, disarticulation, discomfiture,
   discomposure, disconcertedness, disharmony, dishevelment,
   disintegration, disjunction, dislocation, disorder, disorderliness,
   disorganization, disorientation, disproportion, disruption,
   distraction, disturbance, divergence, eccentricity, entropy,
   erraticism, folie, furor, haphazardness, heteromorphism,
   incoherence, indiscriminateness, inferiority, inharmonious harmony,
   insaneness, insanity, irrationality, irregularity, loss of mind,
   loss of reason, lunacy, madness, mania, mental deficiency,
   mental derangement, mental disease, mental disorder,
   mental disturbance, mental illness, mental instability,
   mental sickness, mind overthrown, mindsickness, misarrangement,
   monstrosity, most admired disorder, nonsymmetry, nonuniformity,
   oddness, perturbation, pixilation, possession, promiscuity,
   promiscuousness, psychopathy, queerness, rabidness, randomness,
   reasonlessness, senselessness, shattered mind, shuffling,
   sick mind, sickness, strangeness, subnormality, superiority,
   teratism, turbulence, unbalance, unbalanced mind, unnaturalism,
   unnaturalness, unsaneness, unsound mind, unsoundness,
   unsoundness of mind, unsymmetry, ununiformity, upset,
   witlessness