Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. The state of being besieged; -- used specifically of a
      person beset by a spirit from without. --Tylor.

            Whether by obsession or possession, I will not
            determine.                            --Burton.

Source : WordNet®

obsession
     n 1: an irrational motive for performing trivial or repetitive
          actions against your will [syn: {compulsion}]
     2: an unhealthy and compulsive preoccupation with something or
        someone [syn: {fixation}]