Therasia is a European island.
Great Britain , also known as Britain , is a large island in the north Atlantic Ocean off the northwest coast of continental Europe . With an area of 209,331 km2 , Great Britain is the largest of the British Isles , the largest European island , and the ninth-largest island in the world . Therasia , also known as Thirasía , is an island in the volcanic island group of Santorini in the Greek Cyclades .