Why am I getting ReferenceError: Response is not defined?

I am using node.js with express. I am trying to set up a basic server, but have no idea why I am getting a response error.

var http = require('http');

var myServer = http.createServer(function(req,res){
response.writeHead(200, {"Content-Type" : "text/plain"});
response.write('Hello World');
response.end();

});

myServer.listen(3000);

via Edward K.