The Collaborative International Dictionary of English v.0.48:

Ironwood \I"ron*wood`\, n. (Bot.)
   A tree unusually hard, strong, or heavy wood.
   [1913 Webster]

   Note: In the United States, the hornbeam and the hop hornbeam
         are so called; also the Olneya Tesota, a small tree
         of Arizona; in the West Indies, the Erythroxylon
         areolatum, and several other unrelated trees; in
         China, the Metrosideros vera; in India, the Mesua
         ferrea, and two species of Inga; in Australia, the
         Eucalyptus Sideroxylon, and in many countries,
         species of Sideroxylon and Diospyros, and many
         other trees.
         [1913 Webster]

WordNet (r) 3.0 (2006):

inga
    n 1: any tree or shrub of the genus Inga having pinnate leaves
         and showy usually white flowers; cultivated as ornamentals