A song so good that you are hooked on it the first time you hear it and have to download or buy it it as soon as possible. Once you have it you can happily listen to it on repeat all day long without ever tiring of it, and you feel sick when you haven't heard it for a while.
When it is not being played you just sing or hum it to recreate the feeling you get when you hear the original.
Chris-"Have you heard that Jay-Z song 'Big Pimpin'?"
Luke-"Sure, thats the Heroin Track of the moment, I can't get enough of it"