A song that represents the spirit of summer, and all of the heat and activities that happen during summer. A song that makes you sit up and say, "Man - that is my JAM!"
There is no hard and fast rule that defines a summerjam, but when you hear it, you know.
Rump Shaker may be the quintessential summerjam of the ages.
The Gary Jules version of Mad World is too serious, It's definitely not a summerjam.
It's really easy to point to almost any Jay-Z song and point it out as a summerjam.