Thinner is a solvent used to thin oil-based paints. Commercially, "thinner" is usually a name for mineral spirits.

Other solvents used to thin paint include: Acetone, Mineral turpentine, True turpentine, Naphtha, Toluene, White spirit, Xylene, Methyl ethyl ketone (MEK), and Dimethylformamide (DMF).