Manual:$wgScriptPath

Details
The base path in a URL, used to build URLs to refer to other parts of this wiki. This might be a virtual path, which might be processed using Apache's mod_rewrite (for example), to determine the directory in which MediaWiki assets are found.

''Note: A lot of other variables are defined relative to this one. If you redefine this in your LocalSettings.php then all dependent variables will need checking and might need redefining too!''

This will be set to your base directory in at installation time. See for details.