Diluent
Diluent , adjective
[Latin diluens, p. pr. diluere. See Dilute.]
Diluting; making thinner or weaker by admixture, esp. of water. — Arbuthnot
Diluent , noun
1.
That which dilutes.
2.
(Medicine) An agent used for effecting dilution of the blood; a weak drink.
There is no real diluent but water.