Osmosis is the diffusion of water across a membrane, in fact, osmosis is just simple, passive diffusion. However, since water is so important to life and such an abundant molecule in cells, the diffusion of water has been given its own name - osmosis.
When explaining, discussing and answering questions about osmosis it is super important that you’re able to use the appropriate terminology (and understanding water potential is a fundamental expectation, since osmosis can be quantified using water potential; The short hand symbol for water potential is Ψ the Greek letter psi, pronounced “sy").
Water potential is simply the effective concentration of water which is measured in units of pressure i.e. measured in Pascals (Pa) (or usually kPa kilopascals). The rule is very simple, just like that of passive diffusion:-
“Water always "moves" from a high water potential to a lower water potential”.
The key thing to remember is that 100% pure water has a water potential (Ψ) of zero (0), and zero is the highest possible water potential. Thus, ALL solutions have a water potential (Ψ) Less than zero (0).
You cannot have a water potential (Ψ) great than zero (0).
Another term term you may come across when learning osmosis is osmotic pressure (OP).
You just need to know that the more concentrated a solution, the higher the osmotic pressure. Therefore, OP is the opposite of water potential (Ψ), meaning water will move from a low OP to a high OP.
Now that you know the difference between OP and Ψ you’ll be expected to apply your understanding of osmosis to cells. and once again use the appropriate terminology.
A Level Biology - An Overview of fundamentals of Osmosis