The current HTTP/1.1 specification no longer refers to entities, entity headers or entity-body. Some of the fields are now referred to as representation header fields (RFC 7231, section 3: Representations).
An entity header is an HTTP header that describes the payload of an HTTP message (i.e. metadata about the message body). Entity headers include:
Expires, etc. Entity headers may be present in both HTTP request and response messages.
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