RAD stands for Rapid Application Development.
RAD is a methodology that enables organizations to develop strategically important systems faster while reducing development costs and maintaining quality.
RAD is a programming system that enables programmers to quickly build working programs.
In general, RAD systems provide a number of tools to help build graphical user interfaces that would normally take a large development effort.
Two of the most popular RAD systems for Windows are Visual Basic and Delphi. Historically, RAD systems have tended to emphasize reducing development time, sometimes at the expense of generating in-efficient executable code. Nowadays, though, many RAD systems produce extremely faster code that is optimized.
Conversely, many traditional programming environments now come with a number of visual tools to aid development. Therefore, the line between RAD systems and other development environments has become blurred.
Reference: Information Systems Audit and Control Association, Certified Information Systems Auditor 2002 review manual, chapter 6: Business Application System Development, Acquisition, Implementation and Maintenance (page 307) http://www.webopedia.com