This document defines and explains diffusion, diffusion pressure (DP), and diffusion pressure deficit (DPD). It states that diffusion is the movement of particles from an area of higher concentration to lower concentration until equilibrium is reached. Diffusion pressure is the potential ability of molecules to diffuse, and is directly proportional to concentration. Diffusion pressure deficit is the decrease in diffusion pressure when solutes are added, measured in atmospheres. DPD represents a solution's water absorbing ability and is equivalent to its osmotic pressure.