A paint thinner is a solvent used to dilute oil-based paints[1][2] or varnish. In this context, to dilute is also known as to 'thin'. Paint thinners are diluents. Solvents labeled "paint thinner" are usually white or mineral spirits.
After the paint has lost some of the solvent while in storage, paint can become too thick and dried-out for use.
Some paint thinners spontaneously ignite in relatively low temperature. The most common paint thinner, white spirit, has a very low flash point, at about 40 °C (104 °F),[3] the same as some popular brands of charcoal starter.[4] All such solvents with low flash points are hazardous and must be labelled as flammable.[5][6]