northernmost of Semidi Is.; Aleutian Range. 6 miles long.
name first published as "I(le) Simidin" by Admiral von Krusenstern (1827, map 20), Imperial Russian Navy (IRN), and as "Semidin" by Lieutenant Sarichev (1826, p. 32), Imperial Russian Navy (IRN). "Achaiak" is said to be the Aleut word for the cormorant.