Hidden categories are categories that have the Magic word "__HIDDENCAT__". Hidden categories do not appear on pages (though there's an option in Special:Preferences that allows users to view hidden categories in the editing window), and are most commonly used as maintenance categories.

Pages should not be manually added to this category. They must be added by adding __HIDDENCAT__ or {{Hidden category}} on the category page. The category added can be changed by editing the MediaWiki:Hidden-category-category interface page.