A transparent proxy is one the network puts in your path without you setting it up; on proxy lists the same word means a proxy that passes your real IP along.
Explanation
The word has two meanings, and they are easy to mix up. In networking, a transparent or interception proxy is one you never configured: the router or ISP quietly sends your traffic through it. Hotel and airport Wi-Fi that shows a login page before anything else works this way, and some ISPs use the same trick for caching, filtering or inserting notices. Plain HTTP is easy to intercept. HTTPS is not, because your browser checks the site's certificate, so an interception proxy can usually only see the site name and block or allow the connection, unless the device trusts a certificate it controls, as managed company laptops often do. On free-proxy lists, transparent means something else: a proxy that forwards your real IP address to the site in a header such as X-Forwarded-For. It hides nothing. That is the lowest rung of the transparent, anonymous, elite scale. Either way the lesson is the same: a proxy you did not choose works for whoever put it there, and a proxy labelled transparent does not hide you.
In practice
RFC 3040 calls this an interception proxy. Captive portals are the everyday case, and some ISPs do the same to DNS: Telkom Indonesia's IndiHome has been reported to answer queries sent to any DNS server on port 53 itself, which is why changing to 8.8.8.8 does not get around its filter. A web proxy reached over HTTPS sits on top of all this: the network can see that you connected to the proxy's domain, but not which pages you opened through it.
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