There are plenty of places to go looking for definitions on the Internet. One I like to refer to is MetaGlossary, which tries to extract matching definitions from other web pages.

Enter a term and MetaGlossary returns a list of URLs that provide insight and a definition from the page's text. It's usually quite accurate and provides a great way to look up acronyms, annoying IM abbreviations, or anything else that requires a little clarification. It even does a good job with chemical formulas.

MetaGlossary also provides a Firefox search plugin for quick access to the engine (or you can always just bookmark the search field).