Water crosses a semipermeable membrane to dilute a solute that is unable to cross the membrane. what is this process is called?

The process is called osmosis.

Osmosis is the movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially (semi) permeable membrane.

We know it is osmosis because the solute has a lower water potential and that's the reason why water is needed to dilute it. Plus, osmosis is only the movement of water, but not any other fluid.