An Astian bi-tanle is a stringed musical instrument played by two musicians at once. The Astian bi-tanle was an instrument taught at the Tenaran Music Academy on Trill. (DS9 - The Lives of Dax short story: "Allegro Ouroboros in D Minor") It's unclear whether "Astian" refers to an alien culture, or to some other way in which this particular instrument is distinctive.