DIFFUSION :o

Diffusion has a few meanings.

• The passive movement of molecules or particles along a concentration gradient, or from regions of higher to regions of lower concentration.  
• The spontaneous net movement of particles down their concentration gradient (i.e. difference in the concentrations of substances or molecules between two areas).
• (Cell biology): a type of passive transport, therefore, it is a net movement of molecules in and out of the cell across the cell membrane along a concentration gradient. 

Here's a picture that might help you guys to understand Diffusion better ^^
Reference : http://www.biology-online.org/dictionary/Diffusion
Picture Credits go to owner :)