J.R.R. Tolkien’s fictional realm containing Mordor and Gondor is Middle-earth.
Middle-earth is the main setting for The Hobbit, The Lord of the Rings, and many stories collected in The Silmarillion. It is a region of Tolkien’s wider world of Arda, rather than the name of the entire universe. Mordor, Gondor, Rohan, the Shire, and Mirkwood are all places within Middle-earth.
The term translates the Old English middangeard and related Germanic concepts describing the inhabited world between other cosmic regions. Tolkien’s usage is therefore connected to historical language and mythology, not simply to a generic fantasy map.
Valinor is a blessed land in Aman, west of Middle-earth. Beleriand is an earlier western region described in First Age stories, and Númenor is an island kingdom that existed before its destruction. Those places belong to Tolkien’s legendarium but are not the answer to this question.