Diluent

a. [Lat. diluens, p. pr. of diluere; It. & Sp. diluente. See infra.] Diluting; making more liquid, thinner, or weaker by admixture, especially of water. Arbuthnot.

n. That which dilutes, liquidizes, thins, or weakens any thing by mixture with it; especially, in medicine, a substance used for effecting dilution of the blood.

There is no real diluent but water. Arbuthnot.