The bubonic plague, known as the Black Death, was a bacterial disease spread primarily by fleas carried on rats that devastated Europe beginning in 1347, killing roughly a third of the population. It caused swollen, painful lymph nodes called buboes, along with fever and rapid death, and it spread along trade routes with terrifying speed. The plague's demographic collapse destabilized the feudal economy, triggered labor shortages, and intensified social unrest, making it a central turning point in the book's narrative of the 14th Century.

Chivalry was the code of conduct expected of medieval knights, combining martial skill with ideals of courage, loyalty, honor, and courteous treatment of women and social equals. It was closely tied to the values of the nobility and was reinforced through tournaments, literature, and religious ritual such as the knighting ceremony. The book repeatedly measures the gap between chivalric ideals and the brutal realities of 14th Century warfare, showing how the code often failed to restrain violence and self-interest.

A city charter was a formal grant of rights and privileges issued by a king, lord, or bishop that allowed a town to govern itself, hold markets, and operate under its own laws rather than those of the surrounding feudal estate. Charters typically granted freedom from certain feudal obligations and established municipal councils to manage local affairs. The spread of chartered towns in the medieval period reflects the growth of commerce and the gradual weakening of strict feudal control over land and labor.

A conclave is the closed assembly of cardinals convened to elect a new pope, held under strict seclusion to prevent outside interference. The term comes from the Latin for ‘with a key,’ referring to the practice of locking the cardinals in until a decision was reached. Conclaves during the period covered in A Distant Mirror were often marked by political maneuvering among European powers, a factor that contributed directly to the papal schism the book discusses.

In medieval France, a constable was the highest-ranking military officer of the kingdom, responsible for commanding the royal army and overseeing matters of military justice and discipline. The position ranked above other noble commanders and carried significant political influence at court. Constables appear in the book as key figures directing French forces during the Hundred Years' War and their competence or failure often shaped the outcome of major campaigns.

The dauphin was the title given to the heir apparent to the French throne, roughly equivalent to the English title of Prince of Wales. The title originated from the province of Dauphiné, which was traditionally granted to the king's eldest son. In A Distant Mirror, the dauphin figures prominently during periods of royal incapacity or crisis, when he had to assert authority on behalf of a king who was absent, captured, or unfit to rule.

The First Estate was the clergy, one of the three traditional social orders of medieval France alongside the nobility and the commoners. Members of the First Estate were exempt from most taxation and held significant landed wealth, along with considerable political and spiritual authority. The book uses the estate system to explain the rigid social hierarchy of the period and the tensions that arose when the privileges of the First Estate clashed with the interests of the other two.

The Jacquerie was a violent peasant uprising that erupted in northern France in 1358, driven by anger over the burdens of war taxation, noble exploitation, and the devastation left by military campaigns and plague. Peasants attacked and burned noble estates. The revolt was eventually crushed with brutal reprisals by the nobility. The book presents the Jacquerie as a symptom of the deep social strain fracturing French society in the mid-14th Century.

The Lollards were followers of the English theologian John Wycliffe who challenged core teachings and practices of the medieval Catholic Church, including the authority of the papacy and the doctrine of transubstantiation. They advocated for scripture to be available in the vernacular so ordinary people could read it themselves. Though based in England, the Lollard movement is discussed in the book as part of the broader current of religious dissent that accompanied the institutional crisis of the Church in this period.

A papal bull was an official and formal document issued by the pope, sealed with a lead seal called a bulla, used to declare doctrine, grant privileges, or make significant administrative pronouncements. Bulls carried the full weight of papal authority and were among the most powerful instruments the Church had for shaping political and religious life. The book references papal bulls issued during the period of schism, when rival popes each used such documents to assert their legitimacy.

A poll tax was a fixed tax levied on every individual regardless of income or property, making it especially burdensome for the poor. Such taxes were repeatedly imposed in medieval France and England to help finance the enormous costs of war. The book connects poll taxes to widespread popular resentment, since the flat rate placed a disproportionate burden on peasants and laborers already strained by war and plague.

A schism is a formal split within a religious institution. In the context of A Distant Mirror, it refers specifically to the Great Schism of the Catholic Church beginning in 1378, when rival claimants to the papacy were elected in Rome and Avignon. The division forced European rulers and clergy to choose which pope to recognize, splitting loyalties along political lines. The schism badly damaged the Church's spiritual authority and is treated in the book as a defining crisis of the era.

The Second Estate was the nobility, the hereditary warrior and landholding class of medieval France that held political power, owned vast estates, and was largely exempt from direct taxation. Members of this estate were expected to provide military service to the crown in exchange for their privileges. The book portrays the Second Estate as a class under pressure, its traditional role as protector of the realm undermined by battlefield failures and mounting criticism from below.

A serf was a peasant bound to the land under the feudal system, obligated to work for a lord and pay dues in labor, produce, or money in exchange for protection and the right to farm a plot of land. Unlike enslaved people, serfs could not be bought or sold apart from the land itself, but they lacked personal freedom and required their lord's permission to marry or relocate. The book traces how plague-driven labor shortages weakened serfdom by giving surviving peasants new leverage to demand better terms.

Simony is the buying or selling of church offices, sacraments, or spiritual privileges, a practice named after Simon Magus, who in the Bible attempted to purchase spiritual power from the apostles. It was widely condemned by Church reformers as corrupt because it placed wealth and influence above genuine spiritual calling. A Distant Mirror cites simony as one of the abuses that fuelled criticism of the Church hierarchy during a period already strained by schism and declining moral authority.