ENA variety of names are traditionally used to refer to the literary language as cultivated by the Belarusians and Ukrainians in the late Middle Ages. It is maintained that the emergence of the term prostaja mova / prostyj jazykъ was brought about by the (German) Reformation in the Polish Kingdom and the Grand Duchy of Lithuania. Based on a comparative analysis of the names of the prostaja mova attested in Ruthenian, Polish, and Lithuanian writings, the author surmises that the coinage and the use of the corresponding terms was primarily determined by the revival of the indigenous “linguistic democratism” dating back to the time of Constantine and Methodius. Keywords: Lithuanian; the history of Ukrainian; Belarusian; Ruthenian; prostaja mova; ruskij jazyk; slavenskij jazyk; lingua rustica; die Gemeine Sprache; mowa prosta; język prosty; the Polish Kingdom; the Grand Duchy of Lithuania; Prussia; “linguistic democratism” of Constantine and Methodius. [From the publication]