In the context of search engines, a ban is the most severe form of penalty that can be imposed on a website. This measure is expressed in the complete exclusion of the site from the search engine index, meaning that it no longer appears in search results for any keywords.
Such a penalty is imposed when a site is found to be grossly or systematically violating the officially announced quality guidelines and rules of the search engine. These violations are usually related to the use of deceptive techniques (so-called “black hat SEO”) to manipulate rankings. Examples of such violations include hiding text, cloning content (duplication), or creating an artificial network of links.
A site that has been banned loses virtually all of its organic traffic from the relevant search engine. Lifting the ban usually requires the site owner to first fix all violations, then submit an official reconsideration request to the search engine.