The Collaborative International Dictionary of English v.0.48:

Diluent \Dil"u*ent\, a. [L. diluens, p. pr. diluere. See
   Dilute.]
   Diluting; making thinner or weaker by admixture, esp. of
   water. --Arbuthnot.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diluent \Dil"u*ent\, n.
   1. That which dilutes.
      [1913 Webster]

   2. (Med.) An agent used for effecting dilution of the blood;
      a weak drink.
      [1913 Webster]

            There is no real diluent but water.   --Arbuthnot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diluent
    n 1: a diluting agent [syn: dilutant, diluent, thinner]