!!! Overview
[{$pagename}] is an [HTTP Header Field] describing the content of the [HTTP Body] of the [message]. 

[{$pagename}] are used in both, [HTTP Requests] and [HTTP Responses]. 

[{$pagename}] like [Content-Length], [Content-Language], [Content-Encoding] are [{$pagename}].

Even if [{$pagename}] are neither [HTTP Request Headers] nor [HTTP Response Header], they are often included with these terms.

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Entity header|https://developer.mozilla.org/en-US/docs/Glossary/entity_header|target='_blank'] - based on information obtained 2017-07-12-