HTTP message body

HTTP Message Body is the data bytes transmitted in an HTTP transaction message immediately following the headers if there are any (in the case of HTTP/0.9 no headers are transmitted).
HTTP message.
The request/response message consists of the following:
The request/status line and headers must all end with <CR><LF> (that is, a carriage return followed by a line feed). The empty line must consist of only <CR><LF> and no other whitespace.
The "optional HTTP message body data" is what this article defines.
Response example.
This could be a response from the web server:
HTTP/1.1 200 OK
Date: Sun, 10 Oct 2010 23:26:07 GMT
Server: Apache/2.2.8 (Ubuntu) mod_ssl/2.2.8 OpenSSL/0.9.8g
Last-Modified: Sun, 26 Sep 2010 22:04:35 GMT
ETag: "45b6-834-49130cc1182c0"
Accept-Ranges: bytes
Content-Length: 12
Connection: close
Content-Type: text/html
Hello world!
The message body (or content) in this example is the text "Hello world!".