The Collaborative International Dictionary of English v.0.48:

Neurosis \Neu*ro"sis\, n.; pl. Neuroses. [NL., fr. Gr. ney^ron
   nerve.]
   1. (Med.) A functional nervous affection or disease, that is,
      a disease of the nerves without any appreciable change of
      nerve structure.
      [1913 Webster]

   2. (Psychiatry) a mental or emotional disorder that affects
      only part of the personality, and involves less distorted
      perceptions of reality than a psychosis. As used in
      medicine, anxiety is a prominent characteristic, and the
      condition may be accompanied by psychosomatic symptoms.
      Phobias and compulsive behavior are common varieties.
      --[MW10]
      [PJC]

WordNet (r) 3.0 (2006):

neurosis
    n 1: a mental or personality disturbance not attributable to any
         known neurological or organic dysfunction [syn: neurosis,
         neuroticism, psychoneurosis]