What is Osmosis?

The one definition of osmosis is 'The movement of water from a high concentration to a low concentration, down it's concentration gradient, across a partially permible membrane, until osmotic pressue is equal'. What that essentially means is that water will start to move from one 'place', where there is lots of it and therfore a higher water potential (for example in the blood) to another 'place' (for example where there is less and therefore has a lower water potential until the water potentials in both 'areas' are equal. Osmosis is a form of diffusion just for water and it doesnt use any energy to take place. Therefore we call it a Passive Process.