The idea that terrorism comes in waves was first introduced by David C. Rapoport, the grand old man of terrorism research who started teaching about the subject half a century ago. He distinguished between four waves: the Anarchist Wave (1870-1920s), the Nationalist Wave (1920s-1960s), the New Left/Marxist Wave (from the 1960s to the 1980s), and the Religious Wave (from the late 1970s to today and beyond). Rapoport´s theory is one of the better theories in terrorism research, although it has not gone unchallenged.
Schmid, Alex P. 2016. "Research on Radicalisation: Topics and Themes." Perspectives on Terrorism 10. http://www.terrorismanalysts.com/pt/index.php/pot/article/view/511/html