It's a river: https://en.wikipedia.org/wiki/Tisza
The Tisza or Tisa is one of the main rivers of Central Europe. Once, it was called "the most Hungarian river" because it flowed entirely within the Kingdom of Hungary. Today, it crosses several national borders.
The Tisza begins near Rakhiv in Ukraine, at the confluence of the White Tisa and Black Tisa (the former springs in the Chornohora mountains; the latter in the Gorgany range). From there, the Tisza flows west, roughly following Ukraine's borders with Romania, then Hungary, and finally Serbia. It enters Hungary at Tiszabecs. It traverses Hungary from north to south. A few kilometers south of the Hungarian city of Szeged, it enters Serbia. Finally, it joins the Danube near the village of Novi Slankamen in Vojvodina, Serbia.
There's also a short section in SE Slovakia. Čierna nad Tisou (Čierna on the Tisa) is the railway border station to Ukraine, for example.