Osmosis is the movement of free water molecules through a semipermeable membrane. The water's concentration gradient across the membrane is inversely proportional to the solutes' concentration. Whereas diffusion transports material across membranes and within cells, osmosis transports only water across a membrane, and the membrane limits the diffusion of solutes in the water. Osmosis is a special case of diffusion.

Water, like other substances, moves from a high concentration of free water molecules to a low concentration of free water molecules. Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane, the water level is the same, but there are different concentrations on each side of a dissolved substance, or solute, that cannot cross the membrane.

A principle of diffusion is that the molecules move and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Therefore, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane will continue until the concentration gradient of water goes to zero. Osmosis constantly occurs in living systems.

This text is adapted from Openstax, Concepts of Biology, Section 3.5 Passive Transport.