<p>The database table prefix is a string (a few characters long) prepended to the name of Joomla!'s tables. Using a prefix enables you to run multiple installations of Joomla! using a single database.</p>

<p>The database table prefix can be set during installation. Changing it later is possible, but requires access to the database through a non-Joomla medium or a Joomla Extension such as Akeeba Admin Tools and will cause some downtime.</p>

<p>Extension developers need to use the string <code>#__</code> to represent the prefix. This will be replaced by the real prefix during runtime by Joomla.</p>

<p><strong>Reference: <a  href="http://docs.joomla.org/Glossary" 
        title="read the reference" 
        alt="read the reference" 
        target="_blank"
        rel="nofollow">http://docs.joomla.org/Glossary</a>
</strong></p>