noun

• (European folklore, also figurative) In pre-modern European folklore: an infant of a magical creature that was secretly exchanged for a human infant. In British, Irish and Scandinavian mythology the exchanged infants were thought to be those of fairies, sprites or trolls; in other places, they were ascribed to demons, devils, or witches.
  "[S]he, as her attendant, hath / A louely boy ſtollen, from an Indian king: / She neuer had ſo ſweete a changeling."
• (by extension) A person or object (especially when regarded as inferior) secretly exchanged for something else.
  "When I firſt read Mr. Walker’s Circular Letter to the Arch-deacons, I fancied he intended to give us an Account of thoſe worthy Men, [...] But after about ten Years going with this Work, and that the Time of its Birth was come, I ſoon found it to be a perfect Changeling. Inſtead of what I expected it to be, I ſaw a huge Heap of the moſt Heterogeneous Characters, [...] a great many of them much more deſerving to have their Names blotted out, than their Memory to be preſerved."
• (by extension, informal, rare) An infant secretly exchanged with another infant deliberately or by mistake; a swapling.
• (fantasy, science fiction) An organism which can change shape to mimic others; a shape-shifter.

adj

• (archaic, rare) Changeable, fickle, inconstant, wavering.
  "Away thou changeling motley humouriſt, / Leave me, and in this ſtanding wooden cheſt, / Conſorted with theſe few books, let me lye / In priſon, and here be coffin'd, when I dye."