Manual:$wgDBprefix

Details
Table name prefix. This should be alphanumeric, contain neither spaces nor hyphens, and end in " _ ". If you only have access to a single database and wish to install more than one version of MediaWiki, or have other applications that also use the database, you can give the table names a unique prefix to stop any naming conflicts or confusion.

For example, if you use the prefix 'wiki_' then the table 'page' will be named 'wiki_page' instead.

You need to set this at the time of initial configuration, or manually change the table names at a later date. Changing this setting does not automatically update the names of the tables in your database.

Common value
Many installers set the database table prefix " " to " " by default.