From Tolkien Gateway
Mundburg was the name given to Minas Tirith by the Rohirrim of Rohan in their own tongue. It presumably comes from the Old English mundbeorg, meaning "Protecting Hill". The name of the "Mounds of Mundburg" comes from this title.
Other versions of the legendarium
On an unfinished sketch, Tolkien has written the names Stanburg (Old English) and Steinborg (Old Norse) both meaning "Stone City".