Volatile Vehicle or solvent

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The main purpose of the vehicle is to adjust the viscosity of the paint. It is volatile and does not become part of the paint film. It can also control flow and application properties, and affect the stability of the paint while in liquid state. Its main function is as the carrier for the non volatile components. Volatile substances impart their properties temporarily—once the solvent has evaporated or disintegrated, the remaining paint is fixed to the surface.
This component is optional: some paints have no diluent.
Water is the main vehicle for water-based paints.
Solvent-based, sometimes called oil-based, paints can have various combinations of solvents as the vehicle, including aliphatics, aromatics, alcohols, and ketones. These include organic solvents such as petroleum distillate, esters, glycol ethers, and the like. Sometimes volatile low-molecular weight synthetic resins also serve as diluents. Such solvents are used when water resistance, grease resistance, or similar properties are desired.
Also note that the term "vehicle" is industrial jargon. In some companies the term is used to refer to the solvent and in others, it is used to refer to the binder.