The word osmosis comes from the Greek «ὠσμός» which refers to pushing, impulse. In physics, according to the dictionary of the Spanish Royal Academy, osmosis refers to the passage of solvent, not solute, that occurs between two solutions of different concentrations separated by an impermeable membrane. Then we can define osmosis as a phenomenon of diffusion of water 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 minute that it allows small molecules to pass through the pores, but not large ones which are normally microns in size. An example of this is that it can let water molecules through, since they are small, but not sugar molecules, which are larger.

It is important to make it clear that water is the most abundant molecule inside every individual, and through osmosis it can pass through cell membranes that are semi-permeable to access the cell interior or leave it; 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 carry out 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.