means A being able to alter its physical form at will — swapping human for wolf, fox, bird, or anything else the story needs.

from A transparent English compound of 'shape' and 'shifter.' 'Shape' comes from Old English 'gesceap,' meaning form or creation, tied to the verb 'scieppan,' to create or fashion. 'Shift' descends from Old English 'sciftan,' to arrange, divide, or change. The combined noun is a relatively modern English coinage describing an old, old idea — the creatures themselves (Norse berserkers, Greek Proteus, the fox-spirits of East Asia) are ancient, but the tidy word for them is comparatively young.