What does HTTP Error 451 mean?

What does HTTP Error 451 mean?

• Request timeout
• Unavailable for legal reasons
• Request header fields too large
• Proxy authentication required 

EXPLANATION

HTTP 451: Unavailable For Legal Reasons is an error status code of the HTTP protocol that displays when the user requests an illegal resource, such as a web page censored by a government. 
Examples of situations where an HTTP 451 error code could be displayed include web pages deemed a danger to national security, or web pages deemed to violate copyright, privacy, blasphemy laws, or any other law or court order.
The number 451 is a reference to Ray Bradbury's 1953 dystopian novel Fahrenheit 451, in which books are outlawed.
https://en.wikipedia.org/wiki/HTTP_451