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]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "obsession":
   absorbed attention, absorption, abstraction, abulia, alienation,
   anxiety, anxiety equivalent, anxiety state, apathy, application,
   bedevilment, bewitchery, bewitchment, captivation,
   catatonic stupor, complex, compulsion, concentration,
   contemplation, conviction, deep study, deep thought, dejection,
   depression, detachment, elation, emotionalism, enchantment,
   engagement, engrossment, entrancement, euphoria, fascination,
   fixation, fixed idea, folie du doute, hang-up, hypercathexis,
   hypochondria, hysteria, hysterics, idee fixe, illusion, immersion,
   indifference, insensibility, intentness, involvement,
   irresistible impulse, lethargy, mania, maya, meditation,
   melancholia, mental distress, monomania, morbid drive,
   obsessive compulsion, passion, pathological indecisiveness, phobia,
   possession, preoccupation, prepossession, profound thought,
   psychalgia, psychomotor disturbance, rapt attention,
   ruling passion, single-mindedness, spirit control, studiousness,
   study, stupor, submersion, thing, tic, twitching, unresponsiveness,
   witchery, withdrawal