1. A law of chemistry and physics: the rate of diffusion of one material in another is proportional to the negative of the gradient of the concentration of the first material. 2. A generalization in physiology which states that blood flow is proportional to the difference in concentration of a substance in the blood as it enters and leaves an organ and which is used to determine cardiac output from the difference in oxygen concentration in blood before it enters and after it leaves the lungs and from the rate at which oxygen is consumed. Also known as the Fick principle and Fick method.