It may not have a separate address or stand-alone identity, but Google's glossary search engine is a simple way to look up a word or a group of words in many of the glossaries available online. There are many definitions for "glossary", but one of the best is: "an alphabetical list of words or expressions and the special or technical meanings that they have in a particular book, subject, or activity." (source)
It's difficult to create a comprehensive dictionary with all the words from a language and all of their meanings, so specialized glossaries explain the terminology from a domain (for example: philosophy). There are many glossaries available online, but it's not easy to find all of them using a general-purpose search engine.
Launched in 2002 as a labs project, Google's glossary search engine restricts the index to glossaries and other resources like Wikipedia or WordNet, while allowing you to find all the definitions of a word or expression. To use it, add the define: operator in front of your query: for example, type define:isometry in Google's search box.
The search engine works for 9 languages: English, French, Italian, German, Spanish, Portuguese, Dutch, Chinese, Russian. You can change the language at the bottom of the page.
So when should you use the define operator? If you search for:
* obscure words
* specialized terminology
* acronyms and abbreviations
An interesting alternative is OneLook, which supports pattern matching and reverse queries, but it only shows links to the definitions.