Diffusion is the random movement of a substance from a region of high concentration to a region of low concentration (ie: down the concentration gradient).
For example, oxygen moves from a high concentration in the lungs to a low concentration in the blood.
Substances (oxygen, carbon dioxide, water, dissolved nutrients and urea) can move into and out of a cell or organism via diffusion.
This happens across the cell membrane which is selectively permeable.
The rate of diffusion is affected by the: