An Administrator is an editor who has the extra ability to:
- Block and unblock other users
- Delete and undelete pages on the site
- Protect and unprotect pages
- Edit pages in the MediaWiki namespace (e.g. MediaWiki:Sidebar)
- View deleted edits of a page
- View any contributions by a user to pages which are deleted.
Administrators have basically the same rights as Content administrators, plus the ability to edit site-wide JS/CSS.
Requests for administrator permissions are done via a custom form on Phabricator.