Earworm (noun)

1. A song or melody that runs continually through a person's mind.
2. An idea or phrase that continues to repeat in one's mind.

Origin:

Earworm, coined in the late 19th century, is a calque of the german ohrwurm, which literally means "ear worm" and figuratively means a catchy tune.

Examples:

1. That song is an earworm, I can't get it out of my head.
2. He had an earworm of a jingle stuck in his head all day.
3. I can"t stop thinking about her, it"s like an earworm.
4. She had an earworm of a commercial in her mind.
5. The song became an earworm and was stuck in her head for days.