Ruthenian, or Old Ruthenian (see other names), is a term used for the varieties

Variety (linguistics)

In sociolinguistics a variety, also called a lect, is a specific form of a language or language cluster. This may include languages, dialects, accents, registers, styles or other sociolinguistic variation, as well as the standard variety itself...

 of Eastern Slavonic spoken in the Grand Duchy of Lithuania

Grand Duchy of Lithuania

The Grand Duchy of Lithuania was a European state from the 12th /13th century until 1569 and then as a constituent part of Polish-Lithuanian Commonwealth until 1791 when Constitution of May 3, 1791 abolished it in favor of unitary state. It was founded by the Lithuanians, one of the polytheistic...

 and later in the East Slavic territories of the Polish–Lithuanian Commonwealth.

Scholars do not agree whether Ruthenian was a separate language or a Western dialect(s) of Old East Slavic, but it is agreed that Ruthenian has a close genetic relationship with it.