diluent

That which dilutes.

A solvent or other liquid preparation used to dilute a sample prior to testing.

An agent used for effecting dilution of the blood; a weak drink.

But the case was pressing, and the nearest medical practitioner was sent for. On examining the throat, he expressed his desire that a physician should be called in, and accordingly a gentleman, duly authorized, made his appearance, and prescribed gargles and diluents secundem artem.

Diluting; making thinner or weaker by admixture, especially of water.

there is no real Diluent but Water, every Fluid is diluent as it contains Water in it