diluent

noun

1. That which dilutes.
2. A solvent or other liquid preparation used to dilute a sample prior to testing.
3. 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.”

adj

1. 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”