Geographically speaking, the spread of ideas or knowledge (innovation) from their origins to areas where they can be adopted. Also known as cultural diffusion.

Three Forces working against diffusion:
Distance. The farther from it's source the less likely an innovation will be adopted.
Time. The acceptance of an innovation becomes less likely the longer it takes to reach potential adopters.
Cultural Barriers - Ideas may not be adopted because of prevailing attitudes or taboos.

Diffusion in this sense is becoming more and more obsolete. The reason? Technology. How easy is it to get your idea to people on the other side of the world? Quite easy...just look at Everything2...