What does "ironwood" mean?

Ironwood: A tree unusually hard, strong, or heavy wood. 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.