The Collaborative International Dictionary of English v.0.48:

Buttonwood \But"ton*wood`\, n. (Bot.)
   The Platanus occidentalis, or American plane tree, a large
   tree, producing rough balls, from which it is named; --
   called also buttonball tree, and, in some parts of the
   United States, sycamore. The California buttonwood is
   Platanus racemosa.
   [1913 Webster]