The first person to conduct significant research into the biophysical process of osmosis was Wilhelm Pfeffer in 1877, although the term was first coined in 1854 by Thomas Graham. In order to explain osmosis, however, there are a few key terms to define.
The first is the solvent. This is a liquid, such a water, in which other substances are dissolved. These substances are called the solute, and are solids such as salt or sugar that will break down in the solvent. Once the solute has dissolved in the solvent, we have our final solution. For example, if you were to make a hot chocolate by mixing cocoa powder with boiling water, the water would be the solvent, the cocoa powder would be the solute, and the hot chocolate would be the solution.
The final term we need to understand is the semipermeable membrane. A membrane is a thin sheet of tissue or cells that separates the solution and it is semipermeable because while the solvent can pass through it, the solute cannot.
Osmosis occurs when one compartment, Compartment A, of the solution contains a higher solute concentration than the other. If this happens, the solvent in Compartment B will be drawn into Compartment A through the semi-permeable membrane, ultimately diluting Compartment B.