The term “Litvak” refers to the Jews who lived in “Litva,” the Grand Duchy of Lithuania.  In the late 1700s, the Russian Empire annexed the territory of the Grand Duchy and divided it into several provinces (“gubernyas”).  Since the end of the Second World War, most of the territory that was in the Grand Duchy of Lithuania has been in the modern states of Belarus and Lithuania.

The Baltic Lands, c. 1772    Source:  https://www.litvaksig.org/images/custom/GDL/baltics_1772-UTexas.jpg