A changeling is a creature found in Western European folklore and folk religion. It is typically described as being the offspring of a fairy, troll, elf or other legendary creature that has been secretly left in the place of a human child. Sometimes the term is also used to refer to the child who was taken. The apparent changeling could also be a stock or fetch, an enchanted piece of wood that would soon appear to grow sick and die. The theme of the swapped child is common among medieval literature and reflects concern over infants afflicted by as-then unknown diseases, disorders, or mental retardation.
A human child might be taken due to many factors: to act as a servant, the love of a human child, or malice. Most often it was thought that fairies exchanged the children. Some Norwegian tales tell that the change was made to prevent inbreeding: to give trolls and humans new blood, humans were given children with enormous strength as a reward. In some rare cases, the very elderly of the Fairy people would be exchanged in the place of a human baby, and then the old fairy could live in comfort, being coddled by its human parents. Simple charms, such as an inverted coat or open iron scissors left where the child sleeps, were thought to ward them off; other measures included a constant watch over the child. -Wikipedia

The offspring of a fae and a mortal is called a changeling. They may have odd appearances, depending on their parentage. The offspring of a human and a troll, for example, may appear as a human which is incredibly tall and broadly built with a bulky mass and well-defined jaw. Other changelings have been shown as simply having oddly colored hair, such as green. Changelings must eventually choose whether to become a mortal or a Sidhe. Upon making the choice, the changeling becomes either completely mortal or Sidhe, no longer retaining any of the characteristics of the other half. It is unknown if there is any specific time or age at which a changeling must choose, but during times of war, they are affected by the calling to choose just as the Wyldfae are. -Wikipedia

A changeling is the offspring of a human and a faerie.
At some time the changeling must choose to either become a full human or faerie. Until the time they choose, changelings are under the rule of the Faerie Court of their fae parent. The outward appearance of a changeling depends on their faerie parentage.