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Osmosis is the diffusion of water across a semi-permeable membrane. The "concentration" of water is referred to as water potential. So osmosis is the movement of water from a higher water potential to a lower water potential across a membrane.

For osmosis to occur it is essential that there is a semi-permeable membrane separating two environments with a different solute concentration. The solute must be unable to cross the membrane (molecules too big), but the water molecules are free to pass through and lead to an equilibrium. In the above image, the right side of the beaker has a higher water potential than the left side, so water moves in from right to left.

You must also learn the term isotonic. A solution is isotonic when it has the same water potential as another solution. An example is Ringer's solution which has the same water potential as blood plasma, so can be used to keep tissues alive.