Manual:$wgScriptPath

Details
The base URL path (in relation to DOCUMENT_ROOT). This might be a virtual path (using apache mod_rewrite for example) or the actual directory into which MediaWiki is installed.

''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 redefining too!''

This will be set to your base directory in LocalSettings.php at installation time. See Manual:Short URL for details.