The Broken Arm is the name given to the north-eastern edge of the peninsular of Dorne, where the coastline suddenly breaks apart into the islands of the western Stepstones.
According to legend, the Broken Arm, the Stepstones and the Disputed Lands of Essos used to form a land-bridge linking Westeros to Essos, and across which the First Men migrated. The Children of the Forest are reported to have destroyed the Arm with magic to prevent the First Men's movements, but by the time this had happened, the humans' numbers had become too great.
Today the Broken Arm is held by House Toland of Ghost Hill, answering to the Prince of Dorne in Sunspear.