Lithuania proper refers to a region that existed within the Grand Duchy of Lithuania where the Lithuanian language was spoken. The primary meaning is identical to the Duchy of Lithuania, a land around which the Grand Duchy of Lithuania evolved. The territory can be traced by Catholic Christian parishes established in pagan Baltic lands of the Grand Duchy of Lithuania subsequent to the Christianization of Lithuania in 1387. The Lithuania Proper (Lithuania Propria) was always distinguished from the Ruthenian lands since the Lithuanians differed from the Ruthenians in their language and faith (Paganism in the beginning and Catholicism since 1387). The term in Latin was widely used during the Middle Ages and can be found in numerous historical maps until World War I.

Lithuania proper - Wikipedia