diffusion

Short definition from Wiktionary

Noun

1. 1
   The act of diffusing or dispersing something, or the property of being diffused or dispersed; dispersion.
2. 2
   The scattering of light by reflection from a rough surface, or by passage through a translucent medium.
3. 3
   The intermingling of the molecules of a fluid due to random thermal agitation.
4. 4
   The spread of cultural or linguistic practices, or social institutions, in one or more communities.
5. 5
   The gradual spread and adoption of goods or services.
6. 6
   Exchange of airborne media between regions in space in an apparently random motion of a small scale.
7. 7
   The movement of water vapor from regions of high concentration (high water vapor pressure) toward regions of lower concentration.