The Collaborative International Dictionary of English v.0.48:

Guaiacum \Gua"ia*cum\, n. [NL., fr. Sp. guayaco, from native
   name in Haiti.]
   1. (Bot.) A genus of small, crooked trees, growing in
      tropical America.
      [1913 Webster]

   2. The heart wood or the resin of the Guaiacum officinale
      or lignum-vit[ae], a large tree of the West Indies and
      Central America. It is much used in medicine. [Written
      also guaiac.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lignum-vitae \Lig"num-vi"tae\ (l[i^]g"n[u^]m v[imac]"t[=e]), n.
   [L., wood of life; lignum wood + vita, genitive vit[ae],
   life.] (Bot.)
   A tree (Guaiacum officinale) found in the warm latitudes of
   America, from which the guaiacum of medicine is procured.
   Its wood is very hard and heavy, and is used for various
   mechanical purposes, as for the wheels of ships' blocks,
   cogs, bearings, and the like. See Guaiacum.
   [1913 Webster]

   Note: In New Zealand the Metrosideros buxifolia is called
         lignum-vit[ae], and in Australia a species of Acacia.
         The bastard lignum-vit[ae] is a West Indian tree
         (Sarcomphalus laurinus).
         [1913 Webster]

WordNet (r) 3.0 (2006):

guaiacum
    n 1: medicinal resin from the lignum vitae tree
    2: hard greenish-brown wood of the lignum vitae tree and other
       trees of the genus Guaiacum [syn: lignum vitae, guaiac,
       guaiacum]
    3: small genus of evergreen resinous trees or shrubs of warm and
       tropical America [syn: Guaiacum, genus Guaiacum]