Orthanc is a tower in J.R.R. Tolkien's fantasy world of Middle-earth, built by the Númenóreans. The tower of black stone stands at the center of Isengard's fortifications. It is the residence of Saruman through most of The Lord of the Rings. After the tower is attacked by Ents, Saruman (and Gríma Wormtongue, who arrives at the tower later) are imprisoned within the tower until eventually released by Treebeard.
In Peter Jackson's movie The Lord of the Rings: The Two Towers the two towers referred to are Sauron's Barad-dûr and Orthanc. However, Tolkien's writings leave the matter ambiguous; one other suggestion is that the towers of the title are Orthanc and Minas Morgul on Gondor's border with Mordor.