To understand reverse osmosis, first consider osmosis. Osmosis is the movement of a solvent, such as water, through a semi-permeable membrane to equalize the concentrations of a solute, such as salt, on each side of the membrane. If, for example, unequal concentrations of salt solutions were placed on each side of a suitable membrane, the water would move from the lower concentration side through the membrane to the side with the higher concentration of salt. By applying pressure on the higher concentration side, the process can be reversed, hence reverse osmosis. So reverse osmosis can effectively reduce salt concentrations, and is one of several major approaches used to remove salt from seawater.