DILUENT is a 7 letter word that starts with D.

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

2. \Dil"u*ent\, n.
   1. That which dilutes.
   
   2. (Med.) An agent used for effecting dilution of the blood;
      a weak drink.
   
            There is no real diluent but water.   --Arbuthnot.