What Is Ironwood?

I’ve read several articles over the years and I’m confused as to which species the term “ironwood” actually applies. Can you clarify that for me?

Michael Dresdner: “Iron wood” or ironwood is one of those terms that every area has embraced for one of its hardest local woods. As a consequence, there are over 100 different wood species that, in one area of the world or another, are called ironwood.