Which term describes the movement of water molecules in and out of a cell?

its not diffusion at all, but i think you mean osmosis. it has nothing to do with water concentration so it is a completely distinct mechanism.

osmosis works as the amount of free water decreases as the amount of solute increase. we refer to the proportion of water in this free state as Ψ (psi). solutions with a lower Ψ have more solutes in them and therefore water will travel across a partially permiable membrane down the Ψ gradent, until the amount of free water is the same. the maximum Ψ possible (for comletely pur water) is 0.