'obsession' definitions:

Definition of 'obsession'

(from WordNet)
noun
An irrational motive for performing trivial or repetitive actions, even against your will; "her compulsion to wash her hands repeatedly" [syn: compulsion, obsession]
noun
An unhealthy and compulsive preoccupation with something or someone [syn: obsession, fixation]

Synonyms of 'obsession'

From: Moby Thesaurus