In one sense, what makes something ‘medieval’ is obvious: it is the ‘between times’, the time between the classical and the modern. The term itself means precisely that, via French from from Latin ‘medium’ (middle) and ‘aevum’ (era or period).
But that only puts the question off one level: because what makes something “classical” and what makes something “modern”?
Of the two, defining ‘classical’ is easier. The classical period of a civilisation is the period when it has its first flowering of art, literature, culture and thought; establishing forms of the same that become recurring patterns in that society or civilisation. So ancient Greece and ancient Rome constitute the classical period of Western civilisation. The period of the ruling Caliphate from 642 to the C9th constitutes the classical period of Islam. The Nara and Heian periods the classical period of Japanese history, and so forth.
What makes something “modern”? My preferred answer has been that one is in the modern era when a society experiences a continuing breadth and rate of technological and social change that is discernable to its own inhabitants. The problem with that is that one might say that of of C12th and C13th Latin Christendom: and we would not describe that as “modern”.
The other feature we use when presenting at schools to distinguish a medieval period is one of warrior rule: distinguishing a warrior from a soldier.
A soldier is paid for by taxes: his salary, weapons, equipment and training typically using standard gear in an organised unit. He is, in effect, an armed employee and his watchword is duty: fulfilling the obligations he is paid to undertake. The Roman Army had soldiers. Modern armies have soldiers.
A warrior owns his own weapons, his family probably arranged his training, he is likely to have some direct income source, owes personal service and his watchword is honour: fulfilling the service he has promised to uphold. The knight, the samurai, the Iranian azadan, the Central Eurasian iqta (or similar tax farming fief) holder is a warrior: medieval armies are dominated by warriors.
So the classical period in Islam ends, and the medieval period begins, when the Buyids take power from the Caliphs and begin to distribute iqta fiefs. The classical period of Japan ends, and the medieval period begins, when the bakufu (aka Shogunate), the military government of the warrior clans, is set up so Japan becomes increasingly dominated by samurai politics. Classical civilisation ends when the Western Roman Empire collapses and Germanic kingdoms dominated by warriors owing personal service take over. Developing into the “ultimate” period of warrior rule, knightly Europe.
Hence the modern period begins when warrior rule ends: when rulers put armies of tax-paid soldiers, not personal-service warriors, into the field. This is a process of transition rather than an “on/off” thing. For example, seriously outnumbered English armies won victories such as Crecy, Poitiers, Najera and Agincourt not simply because of the longbow, but because they had much better command-and-control than their enemies. (They had much the same arms mix at Bannockburn, but got horribly beaten because the English command-and-control on the field was crap.) English kings had “cashed out” knights service and used their funds to hire companies from their nobility and gentry on a contract basis: a sort of militant national capitalism. It meant the English King or Prince in charge could give orders he could reasonably expect to be obeyed. In the “personal service” armies of their opponents, command-and-control was much less reliable.
Nevertheless, in European history, the Battle of Fornovo in 1495 is a good marker, since both the Kingdom of France and the Italian League were fielding armies of tax-paid soldiers – recognisably modern armies even though they were still using knights/men-at-arms as heavy cavalry. Certainly, people at the time realised things were changing – hence figures such as Bayard (who famously knighted his sovereign, Francis I) and Maximilian being referred to during their lives as “the last knight”.
The medieval period in Japan ends with the abolition of the Shogunate and the Meiji Restoration of 1867. So, close to four centuries after it does in Latin Christendom, but the medieval period in Japan also starts about seven centuries later than it does in Latin Christendom (and does not occur due to any sort of social collapse).
Trying to date end of the medieval period in Islam is a bit trickier. The abolition of the privileges of the sipahis and their change into cavalry soldiers in 1828 by the Ottoman Sultan is a good marker. Particularly as it comes two year after the “disbanding” slaughter of the Janissaries (known as ‘the Auspicious Incident’: the Janissaries were hated by that time) and 10 years after the massacre of the Citadel ended mamluk power in Egypt. But it is a reasonable question to ask, for example, if Afghanistan has ever entirely got rid of warlords.
Then again, the period of warrior rule in the Scottish highlands probably did not end until the post-Culloden suppressions, including the 1746 abolition of the right of justice of clan chiefs.
So, a medieval period is a period of history between a classical era and the modern period marked by warrior rule – and different civilisations have their medieval periods at different times. While modernity is a period where armies are of tax-paid soldiers and a society experiences a continuing breadth and rate of technological and social change that is discernable to its own inhabitants. Which also does not happen to all civilisations at the same time.
Just because we are all on the same globe does not mean we all enter modernity in the same way and at the same time. After all, various remaining hunter-gatherer societies have still not made it into the Neolithic Revolution.
History happens, but, even in its more general patterns, in its particular ways in particular places.
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