Database Table Prefix
From Joomla! Documentation
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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.
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.
Extension developers need to use the string #__ to represent the prefix. This will be replaced by the real prefix during runtime by Joomla.