Charn is the fictional land ruled by Queen Jadis in C.S. Lewis’s Narnia books.
Readers encounter Charn in The Magician’s Nephew, where Digory Kirke and Polly Plummer arrive after using magical rings. They find an empty, ruined city and awaken Jadis, the last surviving ruler of Charn. Her account suggests that the world was destroyed after she used a devastating spell in a struggle for power.
Jadis later reaches the newly created world of Narnia and becomes the White Witch. Charn is therefore part of the backstory leading to Narnia’s early history, not another name for Narnia itself.
Archenland is a neighboring kingdom in Narnia, while Calormen is a separate empire in the southern part of the Narnian world. Those names are plausible alternatives because they are important locations in the series, but neither is ruled by Jadis in the Charn episode.