Dictionary of Financial Terms
Morgan Stanley - Best for terms in financial markets

Personal Finance Glossary
Mortgage.com - Best for terms in mortgage related terminology

Bankrate Financial Glossary
Bankrate.com - Best for terms related to banking practices

Market Minder
Fischer Investments - Excellent overall reference site with definitions for both 'investment principles' and 'financial terms'

Investopedia
Forbes Media Company - Overall reference grouped into subcategories