A Relational Database Management System (RDBMS) is a Database Management System (DBMS) that is based on the relational model. Most database systems today are based on this type of system.
The term relational database was originally defined by Edgar Codd, a British computer scientist, whilst working at IBM Almaden Research Center. He recognised that the concept of a relational database could be derived from the principles of relational algebra and relational calculus.
http://www.linuxlinks.com/portal/news/article.php?story=20091019024533418