Osmosis is the passage of solvent molecules, generally water, through a semi-permeable membrane from the medium least concentrated in solutes (hypotonic) towards the more concentrated medium (hypertonic).
The phenomenon ceases when the two liquids separated by the membrane reach the same concentration. This is referred to as an isotonic environment. The hydrostatic pressure due to the difference in water level between these two environments then compensates the osmotic pressure.
Illustration of experimental osmosis. © PsYcHoTiK CC by-sa