An innovation or idea that develops in a source area and remains strong there while also spreading outward (the innovation, idea, or disease is moving). Thus, the innovation spreads in a snowball effect so that the total number of adopters and the area of occurrence increases.

There are three types of expansion diffusion:
Contagious Diffusion - a form of expansion diffusion in which nearly all adjacent individuals are affected.
An example of this is the spread of Islam from its hearth on the Arabian Peninsula to Egypt and North Africa, through Southwest Asia, and into West Africa.

Hierarchical Diffusion - the main channel of diffusion is some segment of those who are susceptible to (or adopting) what is being diffused.
An example of this is the spread of AIDS in the United States. Not everyone is affected, only particular vulnerable groups are affected. AIDS doesn't affect in certain areas but instead appear as clusters in distantly separated cities on the map.
Another example is the acceptance of a new style of clothing or new hairstyle. A good number of women adopted the beehive hair-do in the 60's, but not all..

Stimulus Diffusion - a small portion of the population accepts the idea or innovation. This is a local occurance. Some ideas are simply too vague, too unattainable, or too practical for immediate adoption. An example of this is industrialization. During the time of the industrial revolution, only the countries with the capital and the resources were able to industrialize while some lesser developed countries still have not.