diluent

Short definition from Wiktionary

Noun

1. 1
   That which dilutes.
2. 2
   A solvent or other liquid preparation used to dilute a sample prior to testing.
3. 3
   An agent used for effecting dilution of the blood; a weak drink.

Adjective

1. 1
   Diluting; making thinner or weaker by admixture, especially of water.