In networking, a “proxy server” stands between a client and a resource, which is itself a server; it can also fulfill the request of the client on the client’s behalf or even filter or modify the request in a specific way. A “transparent proxy server” performs this task, without the client necessarily being aware of it’s existence.
The Hypertext Transfer Protocol 1.1 definition of a ‘Transparent Proxy’:
“[It] does not modify the request or response beyond what is required for proxy authentication and identification.”
Many proxy server service providers sell ‘Transparent Proxies‘ as a service. In this usage “transparent” is meant in the sense that the client is unaware that the response received originates from the ‘Proxy Server‘ and not from the source server.
A useful example of a ‘Transparent Proxy Server‘ is a gateway, that stands in the middle of client and server and passes through traffic unmodified. This kind of proxy is used in the Simple Mail Transfer System (SMTP).
Another example of a transparent proxy is a reverse proxy, as they distribute incoming traffic directed at a single server to many, this is called ‘load balancing’.