Oh! All right. Thank you. I should have "googled" or "wiki'd" it before opening my mouth. Thank you.
Osmosis is one of a number of phenomena that go by the name of "colligative properties". More here: https://en.wikipedia...ive_properties
- in case you have trouble sleeping at night....
If you have 2 solutions separated by a semi-permeable membrane, so that water molecules can get through it but not those of the substance dissolved, water tends to diffuse from a weaker solution into a stronger solution, i.e. in the direction needed to even out the two concentrations.
The water will continue diffusing from the weaker side to the stronger side until the level of liquid on the stronger side has risen enough to exert a pressure that counterbalances the tendency to diffuse. This is known as the "osmotic pressure".
In reverse osmosis, you exert a pressure, artificially, on the side with the stronger solution, to force water back through into the weaker solution, against this natural osmotic pressure. It's a kind of molecular level filtration, basically.
Osmotic pressure is an important concept in biology. It accounts for how a lot of cellular behaviour works - and even why you find "burst" earthworms on your lawn after rain!