A secret trial is a trial that is not open to the public or generally reported in the news, especially any in-trial proceedings. Generally, no official record of the case or the judge's verdict is made available. Often there is no indictment.
Secret trials have been characteristic of many dictatorships in the modern era, but are also used in many democratic nations, with the explanation of being necessary for national security. They are a hotly debated topic in many circles, but are generally accepted in the western world as they are seen as protecting the "greater good".