The Collaborative International Dictionary of English v.0.48:

Catechu \Cat"e*chu\, n. [See Cashoo.] (Chem.)
   A dry, brown, astringent extract, obtained by decoction and
   evaporation from the Acacia catechu, and several other
   plants growing in India. It contains a large portion of
   tannin or tannic acid, and is used in medicine and in the
   arts. It is also known by the names terra japonica,
   cutch, gambier, etc. --Ure. --Dunglison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

catechu
    n 1: extract of the heartwood of Acacia catechu used for dyeing
         and tanning and preserving fishnets and sails; formerly
         used medicinally [syn: black catechu, catechu]
    2: East Indian spiny tree having twice-pinnate leaves and yellow
       flowers followed by flat pods; source of black catechu [syn:
       catechu, Jerusalem thorn, Acacia catechu]