The term for the diffusion of water across a concentration gradient is osmosis.

Here's a breakdown:

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

* Concentration Gradient: The difference in concentration of a substance between two areas.

* Osmosis: The specific type of diffusion where water moves across a semi-permeable membrane from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration).

Essentially, water moves to dilute the area with a higher concentration of solutes.