What substance is being diffused during osmosis?
Osmosis is defined as the movement of a liquid solvent through a semipermeable membrane from an area of high concentration to an area of low concentration. While the liquid in question could be any solvent, the term osmosis is most closely associated with water. Certainly in any biological-based application the solvent in question is water. A semi-permeable membrane allows water (or a general solvent) to pass through it but does not allow dissolved solute particles to pass through. This allows the liquid solvent to redistribute itself about the membrane and even out its gradient.