The Collaborative International Dictionary of English v.0.48:

Obsession \Ob*ses"sion\, n. [L. obsessio: cf. F. obsession.]
   1. The act of besieging. [archaic] --Johnson.
      [1913 Webster]

   2. The state of being besieged; -- used specifically of a
      person beset by a spirit from without. [archaic] --Tylor.
      [1913 Webster]

            Whether by obsession or possession, I will not
            determine.                            --Burton.
      [1913 Webster]

   3. An excessive preoccupation of the thoughts or feelings;
      the persistent haunting or domination of the mind by a
      particular desire, idea, or image.
      [PJC]

   4. Hence: Any driving motive; a compelling goal; -- not
      necessarily implying a negative judgment, as does sense 3;
      as, the coach was obsessed with winning the state
      championship
      [PJC]

   5. Something that causes an obsession[3].
      [PJC]

   6. The state of being obsessed.
      [PJC] obsessional

WordNet (r) 3.0 (2006):

obsession
    n 1: an irrational motive for performing trivial or repetitive
         actions, even against your will; "her compulsion to wash
         her hands repeatedly" [syn: compulsion, obsession]
    2: an unhealthy and compulsive preoccupation with something or
       someone [syn: obsession, fixation]