An obsession is a persistent, unwanted thought or idea that keeps recurring. For example, a student may be unable to stop thinking that she has neglected to put her name on a test and may think about it constantly for the two weeks it takes to get the paper back. A man may go on vacation and wonder the whole time whether he locked his house. A woman may hear the same tune running through her head over and over.
Obsession is a persistent anxiety-provoking idea that one can't seem to get rid of.