Original: Along with WrestleMania, Royal Rumble and Survivor Series, the event is one of the "Big Four" pay-per-views and was one of the original four annual WWE pay-per-views.
Simplified: Along with WrestleMania, Royal Rumble and Survivor Series, SummerSlam is one of the "Big Four" pay-per-views and was one of the original four yearly WWE pay-per-views.