Here's a breakdown:

* Diffusion: The movement of molecules from an area of high concentration to an area of low concentration.

* Selectively permeable membrane: A membrane that allows some substances to pass through while blocking others. In the case of osmosis, the membrane allows water to pass through but not other solutes.

* Water: The specific molecule that moves during osmosis.

Osmosis is driven by the difference in water potential between two solutions. Water moves from an area of high water potential (where it is more concentrated) to an area of low water potential (where it is less concentrated) in order to reach equilibrium.