dankai no sedai
This phrase was coined by Sekaiya Taichi 堺屋太一 in a novel he wrote in 1976. It literally means 'lump generation' and like the American Baby Boom, it describes the sudden increase of births shortly after WWII (1947-49).
This generation is also referred to as ビートルズ世代 since they came of age during the Beatles craze.
Interestingly, their children (now around 20) are called 団塊ジュニア