Intrusive, persistent thoughts or impulses which provoke anxiety in and cannot be controlled by the person experiencing them.
Example: His obsession with his coworker was beginning to affect his marriage; he ignored his wife and spent hours each day having imaginary conversations with the other woman in his head.
Grinnell, R. (2016). Obsession. Psych Central. Retrieved on February 13, 2018, from https://psychcentral.com/encyclopedia/obsession/