A territory of the Grand Duchy of Lithuania (jointly Polish-Lithuanian Commonwealth from 1569) formed from the bulk of the secularized Livonian Order's lands north of the Daugava, covering present-day Vidzeme, Latgale, and southern Estonia — a Livonian War land-grab that split the old Livonian Confederation three ways alongside the new Duchy of Courland and Semigallia (already in canonical) and Swedish/Danish gains.
The larger, northern part of the Duchy was overrun by Sweden during the Polish–Swedish War (1621-1625), confirmed by the 1629 Truce of Altmark; the Commonwealth retained only the southeastern rump, reorganized as the Inflanty Voivodeship (capital Daugavpils) until absorbed by the Russian Empire in the 1772 First Partition of Poland.
No dedicated English-language monograph specifically on the 1561-1621 Polish/Lithuanian Duchy of Livonia was found on Open Library (searches for 'Duchy of Livonia', 'Polish Livonia' return only unrelated or non-English hits); Plakans' standard scholarly single-volume Baltic history covers this exact polity within its wider Livonian War/Commonwealth-era chapters — used as a verified survey fallback.