A reaction diffusion system is a model where the concentration of a set of chemicals is affected by
- A fixed set of reactions between the chemicals.
- Diffusion of the chemicals. Each chemical will flow from of high concentration areas to low concentration areas.
RD-Systems can model spirals, concentric rings, splotches, and more complex shapes. In The Chemical Basis of Morphogenesis (PDF), Alan Turing proposed that cell differentiaton in embryos was described by this process.