An eigenstrain is a local preferred deformation in the absence of stress. Constraints or incompatibility can make the actual strain differ from it, generating stress. Magnetostrictive strain is one example.
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Eigenstrain is a concept in the field of solid mechanics and material science that refers to a type of internal strain in a material that results from microstructural changes, such as phase transformations, dislocation movement, or other alterations in the material's microstructure, rather than from external loads or boundary conditions. Unlike ordinary strains that occur due to external forces applied to a material, eigenstrains are 'internal' and are typically associated with specific regions or features within the material.