Google have quietly rolled out “Google Dictionary,” which presents definitions and synonyms. In addition to Google's own database of definitions, looking up a word on Dictionary website provides a list of definitions pulled from a variety of academically authoritative sources (Wikipedia and more). You can mark words with the star button and come back to them later, see a list of recent searches. Just go to Dictionary, enter your query and switch to translator resources or dictionaries for other languages -- not to be confused with Google Translate.
Visit: Google Dictionary