When particles move from an area of high concentration to an area of low concentration.
Diffusion can occur in liquids and gasses
Osmosis is a kind of diffusion which only occurs with water.
With osmosis, there is a membrane which does not allow dissolved material to pass though it but only water (selectively permeable) Osmosis is the movement of water molecules from a weak solution to a strong solution.
Osmosis occurs when water is taken up e.g. Plants take up water from the ground.