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An entity header is an HTTP header describing the content of the body of the message. Entity headers are used in both, HTTP requests and responses. Headers like Content-Length, Content-Language, Content-Encoding are entity headers.
Even if entity headers are neither request nor response headers, they are often included with these terms.
In the following example, Content-Length is an entity header, while Host and User-Agent are request headers:
POST /myform.html HTTP/1.1 Host: developer.mozilla.org User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0 Content-Length: 128