The Collaborative International Dictionary of English v.0.48:

Duramen \Du*ra"men\, n. [L., hardness, a hardened, i. e.,
   ligneous, vine branch, fr. durare to harden. See Dure.]
   (Bot.)
   The heartwood of an exogenous tree.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Heartwood \Heart"wood`\ (h[aum]rt"w[oo^]d`), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

duramen
    n 1: the older inactive central wood of a tree or woody plant;
         usually darker and denser than the surrounding sapwood
         [syn: heartwood, duramen]