Is osmosis diffusion of water?

Yes, In most cases, osmosis is the diffusion of water. To my understanding, what I think Wikipedia is trying to do is to assert the fact that osmosis is also be the movement of water from a region of lower water potential to a region of higher water potential. These kinds of contradictions, as much as scientists try to avoid them, still come to play because science is very complicated. Just when we start accepting a theory, it gets discarded by another theory.

Osmosis is about solutions (solvent + solute).

Osmosis is a movement of the solvent through certain types of membranes. These membranes let the solvent move across them, but not the solute. The process tends to balance the solute concentration on both sides of the membranes.

If we say that the solvent is water and the solute is salt (NaCl). Water will move from the side of the membrane where the salt concentration is low toward the side where the salt concentration is high. At the end of the process both concentrations will be equal.

In biology it is a very important process at a cellular level (cell membranes are semi-permeable membranes) but also plays a key role in food digestion or kidney reabsorption, for instance. Cells (bacteria) that are put in solutions with high concentration of salt die of loosing the water they contain.