Why did eighteenth century marked the beginning of medieval period?

The eighteenth century did not mark the beginning of the medieval period; rather, it is typically associated with the Enlightenment and the transition from the early modern period to modernity. The medieval period, or Middle Ages, traditionally spans from the 5th to the late 15th century, following the fall of the Western Roman Empire and preceding the Renaissance. The confusion may arise from discussions of historical periods, but the eighteenth century is characterized by advancements in science, philosophy, and political thought that diverged from medieval traditions.