Manual:$wgSitename

Details
The name of the site, as displayed throughout the site. Should always be changed from the default setting.

This name gets included many times throughout the system. For instance, the Wikipedia tagline "'From Wikipedia, the free encyclopedia."' makes use of this setting.

If $wgMetaNamespace is not set, it will be automatically initialized to this value. Make sure the name is appropriately formatted: valid title characters, etc.