Possibly because you know that one of the 316 improvements that Apple has included in Leopard (Mac OS X 10.5.x) refers to the implementation Dictionary. In this application, perfectly integrated into the operating system, includes new dictionaries.
It is pleasantly surprising, and really useful, enter the New Oxford American Dictionary, or Wikipedia1 - among other works-through this program, the Dictionary widget or any Cocoa application software such as Safari or Spotlight search.