Reverse Proxy

E.g. NGINX
http://en.kioskea.net/contents/308-proxy-and-reverse-proxy-servers

A reverse-proxy is a "backwards" proxy-cache server; it's a proxy server that, rather than allowing internal users to access the Internet, lets Internet users indirectly access certain internal servers.

http://serverfault.com/questions/8654/what-is-a-reverse-proxy

A reverse proxy, also known as an "inbound" proxy is a server that receives requests from the Internet and forwards (proxies) them to a small set of servers, usually located on an internal network and not directly accessible from outside. It's "reverse", because a traditional ("outbound") proxy receives requests from a small set of clients on an internal network and forwards them to the Internet. 

http://www.jscape.com/blog/bid/87783/Forward-Proxy-vs-Reverse-Proxy
         has good diagrams to differentiate between a forward-proxy and a reverse-proxy