Final answer:
Expansion diffusion is the spread of a feature or trend among people from one area to another in a snowballing process. It can occur through contagious diffusion or hierarchical diffusion and can be seen in the spread of religions like Islam.
Step-by-step explanation:
Expansion diffusion refers to the spread of a feature or trend among people from one area to another in a snowballing process. It can occur through two main subtypes: contagious diffusion and hierarchical diffusion. Contagious diffusion happens when a feature or trend spreads from one point and expands outwardly, similar to the spread of a disease. Hierarchical diffusion, on the other hand, occurs when a feature or trend is adopted by rulers or authority figures and then filters down to the masses. This type of diffusion can be seen in the spread of religions like Islam.