The Collaborative International Dictionary of English v.0.48:

Guaiac \Gua"iac\, a. [See Guaiacum.]
   Pertaining to, or resembling, guaiacum. -- n. Guaiacum.
   [1913 Webster]

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]

WordNet (r) 3.0 (2006):

guaiac
    n 1: hard greenish-brown wood of the lignum vitae tree and other
         trees of the genus Guaiacum [syn: lignum vitae, guaiac,
         guaiacum]