eReference for Mac

From DedicatedTeacher.com:

Want to know the meaning of "efficacy" at the same instant you're reading it on a Web page or in a Microsoft Office application? Houghton Mifflin eReference will find the answer after you've simply highlighted the word and clicked on the "A-Z" icon. Want to get a definition of a word, but you're not sure of the spelling? Start typing your best guess and Houghton Mifflin eReference will generate a list of possible matches. How about then getting a synonym or antonym of "efficacy"? With Houghton Mifflin eReference, you can quickly toggle between the Dictionary and Thesaurus.