Diffusion is the net movement of a substance from a region of high concentration to a region of low concentration. The difference between concentrations in a space is a "concentration gradient." As explained by The Biology Corner, molecules will always move down a concentration gradient toward areas of lesser concentration.
From its Latin word origins, "diffusion" means "to spread out." In biological organisms, certain membranes provide a selectively permeable barrier, which allows water and oxygen to move freely across the membrane by diffusion. The diffusion of water across a membrane is osmosis. Water moves in the direction of high solute concentration, where the water concentration is therefore lower. Diffusion is a type of passive transport, as it does not require the input of chemical energy, unlike active transport.