(EER-werm), from the German ohrwurm
: a song whose tune and/or words resonate in one's mind even in the absence of the song. Also connotes the tune's ability to spread from host to host via almost any audio vector
Howard Rheingold, in his article "Untranslatable Words" (which appeared in the 1987 Whole Earth Review), appears to have been the first to set the English version of this word in print. It shouldn't come as a surprise that the Germans, with their viciously precise language full of laser-guided compound words, and their pop culture bred from a bizarre marriage of Cold War socialism and capitalism, invented the term ohrwurm. Ever since Beethoven and Mozart, the Germans have known catchy music whenever they heard it. Like the Mexican phrase tecato gusano (approx. "the junkie's worm"), it just feels right to describe an external influence that has become internal as a "worm". Similarly, by analogy to the computer term "worm" (for malicious code which spreads quickly under the surface) you are a victim and a vector when you have an earworm.
It doesn't take a brain surgeon to recognize the phenomenon: you hear a song, or worse, a snippet of a song, and it's etched into your brain for the next five or six days. You hum it in rhythm with your breathing as you go for a run, you whistle it on the walk across the street for lunch. It follows you, sticks with you, infects you. Paradoxically, you feel the urge to listen to it some more, as though finishing the song would complete your sentence and release you from its spell. Often this serves only to burn it further into your brain. Singing it out loud (provided the tune is recognizable) infects others, and they stumble along like zombies, your audible meme replacing their grocery list in their head, overwriting critical bits of data, jamming their reception of anything else. It competes for processor cycles. It is insidious.
It doesn't take a scientist, but that didn't stop the researchers at Dartmouth University. They discovered (and reported in the journal Nature) that the brain's auditory cortex goes into high gear when you listen to a song, especially one that you know well. Interestingly enough, they also discovered that if the music stops, the auditory cortex keeps "singing along" for several seconds. The effect was different depending on whether or not the song had lyrics, but in either case the brain would continue to sing the song. It seems our brain has evolved to record and play back music, as though hearing the next measure of "Love Shack" or "YMCA" would somehow provide us with just the right data to hunt down that woolly mammoth. "Yes, of course! I'll hit it with a tin roof -- rusted." The familiarity of a song seems to play a strong role in the phenomenon; unfamiliar songs did not seem to cause the same effect. One might hypothesize that the brain needs to have already "recorded" the song in its entirety before it can effect a "playback" in the song's absence. No word on how many times one has to hear a song to record it.
For completeness' sake, it is worth noting that the songs they used to test this theory included the Rolling Stones' Satisfaction and Henry Mancini's The Pink Panther.
Da-dum, da-dum, da-dum-de-dum-de-dum, da-dum-de-daaaaa...