In the history of archaeological theory the term migrationism was opposed to the term diffusionism (or “immobilism”) as a means of distinguishing two approaches to explaining the spread of prehistoric archaeological cultures and innovations in artefact. Migrationism explains cultural change in terms of human migration, while diffusionism relies on explanations based on trans-cultural diffusion of ideas rather than populations (pots, not people).
Western archaeology the first half of the 20th century relied on the assumption of migration and invasion as driving cultural change. This was criticized by the processualist in the 1960s and 1970s, leading to a new mainstream which rejected “migrationism” as outdated. Since the 1990s, there has been renewed interest in “migrationist” scenarios, as archaeologists attempted the archaeological reflexes of migrations known to have occurred historically.
Thanks to Cliff Dunning http://www.earthancients.com/