The word osmosis comes from the Greek "ὠσμός" which refers to pushing, impulse. In physics according to the dictionary of the Royal Spanish Academy, osmosis refers to the passage of solvent, not solute, which occurs between two solutions of different concentration separated by an impermeable membrane. So we can define osmosis as a phenomenon of water diffusion through a semi-permeable membrane, this is one that has pores, similar to any filter of molecular size. The dimension of these pores is so tiny that it allows small molecules to pass through the pores, but not the large ones that are normally the size of microns. An example of this is that it can let the water molecules pass since they are small but not the sugar molecules that are larger.
It is important to make clear that water is the most abundant molecule in the interior of every individual, and by means of osmosis it can pass through cell membranes that are semi-permeable to enter or leave the cell; this depends on the difference in concentration between intracellular fluids and extracellular fluids, which are determined by the existence of dissolved organic molecules and mineral salts.
On the other hand there is reverse osmosis, this occurs when a pressure higher than the osmotic pressure is used, and this is when the opposite effect occurs; when fluids are pressed through the membrane, thus leaving dissolved solids behind. In the water purification process, for example, we need to perform reverse osmosis, that is, the opposite of conventional osmosis. In this process, in order to force the passage of the water found in the brine stream to the stream of water with low salt concentration, it is necessary to pressurize the water to a value higher than the osmotic pressure; and due to this process the brine becomes more concentrated.
