The "Long Fourteenth Century" is based around the period of time during which the typical conditions of the Fourteenth Century persisted beyond the strict limits of the century. In this case "from the later decades of the 13th century to the middle decades of the 15th century" according to one definition I found. This is an increasingly common approach in history to look at Medieval or Early Modern centuries as historical phases of substance that often extend beyond the automatic assortment of the centuries themselves. So we also get the Long Sixteenth Century in English history extending until Charles I, because James I's reign had more in common with his Tudor predecessors than the later Stuarts.

Sometimes this "Long Century" concept is applied in various countries, even in different continents. Other times it is more relevant in some countries than others. Consider, for example, the way we define "The Victorian Period" vs "Qing Dynasty". So it's important to consider what the best way of divvying up time is within your given context, it is not necessarily the same in different places.