Solvent is a liquid that dissolves another substance to form a solution. In the process of making paints, a solvent is used to dissolve resins so that pigments are mixed easily with the resins. A thinner is a mixture of solvents. It is used to dilute paints to an appropriate viscosity level for application. Solvents and thinners evaporate as the paint is dried, and do not remain in the coating.
Because of this, the specific blends of several types of solvents are required for specific paint types. In addition, reducer/thinners are available in different evaporation rates to compensate for ambient temperatures & conditions.