A song or tune that continues playing inside one's head, often to the intense annoyance of the particular head concerned. The song/tune will typically be of the hooky variety, making it easy to absorb and remember on just one or two hearings. Unfortunately for sufferers, it is often the most trite and irritating parasites (i.e. melodies) which attach to the host ear...
This was first coined, I believe, in Germany. The german translation is "Ohrwurm".
"Oh shit! I've got this earworm in my head and it's playing Kylie songs!"