Mainframe platform aren't part of a unified platform, but playable on one or more Mainframe or Minicomputers. Mainframes are large expensive computers that have been around since just after World War II and are generally much more powerful than computers people use in their houses. They generally aren't created to play games on, but many games have been created to run on these computers.
This platform is closely tied to the Terminal platform, as usually a mainframe is accessed through an external Terminal. In most cases we will document such games under the Mainframe platform. Notable exceptions are if games are run over larger networks such as PLATO.
Games that were released in a standard BASIC programming language format are also documented under the Mainframe platform as they are likely to be playable on most Mainframe and Mini computers as well as a large number of micro computers.