Obsession (noun)

1. An unhealthy and compulsive preoccupation with a persoan, object or idea.
2. An idea or feeling that continually preoccupies or intrudes on a person's thoughts.

Origin:

From latin obsessio, from obsessus, past participle of obsidere "to besiege".

Examples:

1. His obsession with cleanliness borders on the obsessive-compulsive.
2. She had an obsession with her appearance.
3. He developed an obsession for collecting old coins.
4. The musician had an obsession for playing his instrument.
5. He had an obsession with a girl he met in college.