A concentration gradient occurs when there is a difference in concentration of a particular substance between two regions. Substances will naturally move from areas of high concentration to areas of low concentration until equilibrium is reached.
A special type of diffusion involving water molecules moving across a semi-permeable membrane from an area with more water (low solute concentration) towards less water (high solute concentration).
Equilibrium: The state where concentrations are equal throughout a space; no net movement occurs once this state has been reached.