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Retained austenite is the face-centered-cubic gamma phase of iron or steel that remains present in the room-temperature microstructure after a heat treatment or processing cycle that, under equilibrium, would have transformed it fully into a room-temperature product such as martensite, bainite, or ferrite ^[1]. The phase is retained because the martensitic or diffusional transformation is suppressed before completion, typically through chemical stabilization by dissolved carbon and alloying elements, through insufficient undercooling below the transformation temperature, or through mechanical stabilization, so that a fraction of the high-temperature austenite survives at ambient temperature in a thermodynamically metastable state ^[2]. Retained austenite is distinguished from equilibrium austenite by its metastable, non-equilibrium nature: it persists only because the transformation that should have occurred was kinetically or thermodynamically arrested, and it may subsequently transform to martensite upon cooling below its martensite-start temperature or upon plastic deformation ^[1]. Its volume fraction, composition, and stability govern its influence on mechanical behavior, because the phase is softer than martensite yet capable of strain-induced transformation, and its identification requires the distinction between stabilized surviving austenite and products that formed in its place ^[2]. It is a constituent of multiphase steels whose amount and stability are controlled by alloying and thermal history ^[3].