Apparently they didn't have one. Now they do:
The search giant has quietly rolled out Google Dictionary, which presents definitions and synonyms. Exactly what you'd expect from a dictionary.
In addition to Google's own database of definitions, looking up a word on the Dictionary website provides a list of definitions pulled from a variety of academically authoritative sources (oh, and Wikipedia).
It's odd that the Monster of Mountain View was relying on Answers.com for definitions of search queries previously. Google doesn't like to outsource… their goal is to have a product for everything.
Dictionary.com, which is owned by the same company that owns Ask.com, will remain superior to “Google Dictionary”.