Concentration refers to how much solute is present compared to solvent. In other words, it's a measure of how much of one substance (the solute) is dissolved in another (the solvent).
A unit of concentration, defined as the number of moles of solute per liter of solution.
The maximum amount of solute that can dissolve in a solvent at a given temperature.
The process by which the concentration of a solution is reduced, usually by adding more solvent.