The Basilisk Isles are a large island chain off the northern coast of the continent of Sothoryos, south of Slaver's Bay and Essos. One of the few mapped portions of Sothoryos, they are in the extreme south of the mapped world known to the inhabitants of the Seven Kingdoms of Westeros. Asshai and Yi Ti are rumored to be even further due east, but they are unmapped. The island of Naath, also part of Sothoryos, is located west of the Basilisk Isles chain.
When an assassination attempt is made on Daenerys Targaryen in Vaes Dothrak, she says she thought King Robert Baratheon would leave her alone after her brother Viserys Targaryen died. Ser Jorah Mormont then explains to her that as the last Targaryen, King Robert's agents will never stop hunting her no matter how far she flees, even if she goes to Asshai or the Basilisk Isles.
In the books
In the A Song of Ice and Fire novels, the Basilisk Isles are the same. To the south lies Zamettar, in the Sothoryos mainland, to the east Ax Isle and to the southwest, the Isle of Toads.