Use define:
in Google and you will get dictionary definitions and etymologies. For instance: define: dictionary.
Where is Google getting the definitions, etymologies, and pronunciations? I doubt they are compiling it themselves—they probably license it from somewhere.
Best Answer
Presumably a multitude of sources (for different languages). For me (English - not American!) inspection of the source for a search result shows mention of www.oxforddictionaries.com.