What is the name of the tower of Isengard?

What is the name of the tower of Isengard? The tower of Isengard is called Orthanc. It stands at the center of the circular fortress complex known as Isengard in J. R. R. Tolkien's Middle-earth.

Orthanc was built by the Númenórean exiles who founded Gondor, toward the end of the Second Age. The tower was formed from four immense, many-sided columns of dark stone joined by an apparently seamless process. Its height and unusual construction made it one of the most remarkable structures in the region.

During the War of the Ring, the wizard Saruman occupied Orthanc and used Isengard as a military base for breeding and equipping Orcs. The Ents later attacked and flooded the surrounding fortress, but Orthanc itself resisted their efforts to destroy it. A common mix-up is to treat Isengard and Orthanc as the same place: Isengard is the entire fortress, while Orthanc is its central tower.