The term "Ruthenia" may mean significantly different things, dependent on time period and subject. The word "Ruthenia" is a Latin rendering of the ancient place name "Rus".
In European manuscripts dating from the 13th century CE, "Ruthenia" is used to describe lands of the Ancient Rus, ie, parts of Eastern Europe.
Today, the historical territory of Rus is formed with part(s) of the lands of Belarus, northern Ukraine, western Russia, small parts of northeastern Slovakia, and narrow strips of eastern Poland.
Read more about Ruthenia: Early Middle Ages, Late Middle Ages, Cognate Word, Fictional Usage