A song with an 'addictive' element (hook, melody, lyric) that draws people into listening multiple times. Song doesn't have to be good or bad, it just needs to have that 'addictive' element.
'Hey Ya' is frontrunner for earworm of the year, Outkast shouldn't make such addictive songs :D.
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...
Etymology:
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!"
A song that won't go away, and gets stuck in your head. Makes you want to tear your ears out; unfortunately, deafness can't cure it because it's in there for good.
Oh, I don't know... "Seasons in the Sun", "Muskrat Love", "Lovefool"...