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A diluent (also referred to as a filler, dilutant or thinner) is a diluting agent.
Certain fluids are too viscous to be pumped easily or too dense to flow from one particular point to the other. This can be problematic, because it might not be economically feasible to transport such fluids in this state.
To ease this restricted movement, diluents are added. This decreases the viscosity of the fluids, thereby also decreasing the pumping/transportation costs.
One industrial application is the transport of crude oil via pipelines. Heavy crude oil/bitumen are fluids with high viscosity, especially at low temperatures. The addition of a diluent enables the diluted fluid (dilbit in the case of bitumen) to meet pipeline specifications in order for it to be efficiently transported. Typical diluent in this case is naphtha or condensate.
Types of diluents more familiar to the general public include paint thinner and nail polish thinner, both of which improve the consistency and applicability of the products to which they are added.
Diluent is also used as a term in solvent extraction for an inert solvent in which a metal extraction agent (extractant) is dissolved in. In solvent extraction the diluent has potentially several uses. It can be used as a solvent (in the purely chemical sense rather than the solvent extraction sense) to dissolve an extractant which is a solid and so render it suitable for use in a liquid–liquid extraction process. In other cases such as PUREX nuclear reprocessing the diluent (kerosene) is used to reduce the maximum metal loading which the organic layer can reach. If the organic layer was to acquire too much metal then a solid metal complex might form, or more worryingly in a nuclear process the potential for a criticality accident if the fissile metal concentration in the organic phase becomes too high.
Uses of diluents in pharmaceuticals 
Diluents are also very important in the pharmaceutical industry. They are inactive ingredients that are added to tablets and capsules in addition to the active drug. For example, a Tylenol 325 mg tablet does not weigh 325 mg. This is the weight of the active acetaminophen, while the tablet weighs more due to other additives known as diluents. These additives may be used as binders, disintegrants (help the tablet break apart in the digestive system), or flavor enhancers. Some very common diluents in tablets include starch, cellulose derivatives, and magnesium stearate (a lubricant). (See also Excipient.)
Diluents are also used in vaccines to reconstitute the vaccine after storage, such as MMR.