The very obvious

ob·ses·sion /əbˈsɛʃən/ Show Spelled[uhb-sesh-uhn] 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.

Origin:
1505–15; < class="ital-inline">obsessiōn- (s. of obsessiō) blockade, siege, equiv. to obsess(us) (see obsess) + -iōn- -ion

—Related forms

ob·ses·sion·al, adjective

non·ob·ses·sion, noun

non·ob·ses·sion·al, adjective

self-ob·ses·sion, noun