In folklore, babies or children secretly switched for human infants by fairies or supernatural beings; or figuratively, anyone who seems different from their family or origins.
From 'changeling' (from 'change' + the suffix '-ling', indicating a person or creature involved in that action), originally from medieval European folklore. The term combines 'change' with '-ling' (a person of small importance or a child).
Changelings in folklore were probably ancient ways of explaining why some children had disabilities or seemed 'different'—tragic, since it meant those children were sometimes mistreated, yet the mythology also shows humans have long been puzzled by human diversity.
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