The American Heritage® Dictionary of the English Language, Fourth Edition

noun

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

Dictionary.com Unabridged (v 1.1) – Cite This Source
ob·ses·sion / Pronunciation Key – Show Spelled Pronunciation[uhb-sesh-uhn] Pronunciation Key –

–noun 1.
the domination of one’s thoughts or feelings by a persistent idea, image, desire, etc.
2. the idea, image, desire, feeling, etc., itself.
3. the state of being obsessed.
4. the act of obsessing.

American Heritage Dictionary – Cite This Source ob·ses·sion (?b-s?sh’?n, ?b-) Pronunciation Key
n.

Compulsive preoccupation with a fixed idea or an unwanted feeling or emotion, often accompanied by symptoms of anxiety.
A compulsive, often unreasonable idea or emotion.

ob·ses’sion·al adj., ob·ses’sion·al·ly adv.