Osmosis:

Osmosis is a process of movement of water molecules from a region of higher concentration to a region of lower concentration, down the concentration gradient, using a semipermeable membrane.

Like diffusion, osmosis is also the movement from higher concentration to lower concentration, it is also regarded as a type diffusion. Solute molecules are large enough, so due to the limited space in the cell surface membrane, solute molecules cannot move. Cell membrane is a partially permeable membrane, that means, it allows few molecules to pass through and some not. Solute molecules are confined within space.