Vehicle
The liquid part of a paint that carries the pigment: the binder plus any thinner.
Also: paint vehicle
The vehicle is everything in the paint that is not pigment. It carries the pigment from the brush to the surface. It usually has two parts: the binder, which stays behind as a film, and a solvent or thinner, such as water or turpentine, which evaporates as the paint dries. In watercolour, the vehicle is gum arabic dissolved in water. People often use vehicle, medium and binder as if they meant the same thing, and for most purposes little is lost.