HTTP 451 - Definition from the Hotspot Shield Glossary
HTTP 451 is a Hypertext Transfer Protocol error message that displays “HTTP 451 Unavailable for Legal Reasons”. If a http web page cannot be displayed, an error message will be displayed that will contain a code and a message - Other codes include 404 (Not found) and 403 (Forbidden).
HTTP error message 451 is used when a website cannot be accessed due to legal reasons i.e. a government has censored it or it contains material that infringes on copyrights and laws. The use of HTTP 451 varies from country to country and some nations will opt to use the standard error 404 instead to simply state the website has not been found, as opposed to explicitly stating that it is unavailable for legal reasons.