DECOMPOUND is a 10 letter word that starts with D.

1. \De`com*pound"\, v. t. [imp. & p. p. {Decompounded};
   p. pr. & vb. n. {Decompounding}.] [Pref. de- (intens. in
   sense 1) + compound, v. t.]
   1. To compound or mix with that is already compound; to
      compound a second time.
   
   2. To reduce to constituent parts; to decompose.
   
            It divides and decompounds objects into . . . parts.
                                                  --Hazlitt.
   
   

2. \De`com*pound"\, a. [Pref. de- (intens.) + compound,
   a.]
   1. Compound of what is already compounded; compounded a
      second time.
   
   2. (Bot.) Several times compounded or divided, as a leaf or
      stem; decomposite.
   
   

3. \De`com*pound"\, n.
   A decomposite.