A changeling is an elf who was left (as an infant) with a lowfolk foster-family. Usually the changeling is secretly exchanged for the lowfolk family's actual child, which is taken back to Faerie. These stolen lowfolk children are known as "swaplings."
Elves leave their children as changelings for a variety of reasons. They may be too busy or too lazy to take care of their own children. It was rumored that Unseelie elves of the Long Ago stole children to eat or sacrifice (leaving their own offspring as "fair trade") but these practices are illegal in the Mephitist Empire. In the later Imperial era, elves of the aristocracy would send their offspring away as changelings because it was a traditional mark of prestige, and was believed to "build character." During the reign of Adler Sartoriusson it became mandatory for each family to send at least one child away as a changeling. During the reign of Gawain Adlersson, returning changelings had to pass through the Ferifax Arch in a ceremony that bound a swapling's spiritual essence to them, producing a hybrid creature called a "crossling."
Many changelings do not realize they are elves until they reach a certain age, upon which they are visited by elvish agents, or are mystically "called" to return to Faerie by the nearest Gate.