A substance that is added to dilute a solution or mixture; to reduce the strength of a solution by the addition of water or other solvent. Diluents are used to alter the viscosity of a solution in order to meet specifications, such as the addition of naphtha or condensate to heavy oils to meet pipeline specifications for transportation. In solvent extraction, diluents are inert solvents such as kerosene in the purex process to control the extraction capacity of the organic layer.