Osmosis is not the fusion of water across a membrane. Osmosis is the net movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration, in an attempt to equalize the solute concentrations on both sides.

The term "fusion" implies that two or more things are coming together to form a single entity. In osmosis, water molecules are moving from an area of lower solute concentration to an area of higher solute concentration. This movement of water molecules is not akin to the fusion of water molecules.