The social, economic, and cultural order that developed in Europe after the fall of the Western Roman Empire — organized around feudalism, the Catholic Church, and the manorial economy — before being transformed by the Black Death, the Renaissance, the printing press, and the Reformation into the early modern world.
The medieval period is the era that modernity defined itself against — 'dark ages' of superstition, feudal oppression, and intellectual stagnation that the Renaissance and Enlightenment claimed to have escaped. The reality was more interesting. Medieval Europe invented the university, the gothic cathedral, the common law, parliamentary representation, and double-entry bookkeeping. It also burned heretics, persecuted Jews, and organized crusades. The Middle Ages is the world we came from, not the world we escaped.