Source : Webster's Revised Unabridged Dictionary (1913)

Heartwood \Heart"wood`\, n.
   The hard, central part of the trunk of a tree, consisting of
   the old and matured wood, and usually differing in color from
   the outer layers. It is technically known as duramen, and
   distinguished from the softer sapwood or alburnum.

Source : WordNet®

heartwood
     n : the older inactive central wood of a tree or woody plant;
         usually darker and denser than the surrounding sapwood
         [syn: {duramen}]