What is defined as the maximum stress a material can withstand without permanent deformation?

The concept being described is the maximum stress a material can withstand without permanent deformation, which aligns with the term "ultimate strength." Ultimate strength refers to the maximum stress a material can experience before it begins to fail, which includes both yielding and breaking points. This property is crucial in materials science and engineering, as it helps in determining how much load a material can handle in various applications without changing its shape permanently.

Tensile testing refers to the method used to find ultimate strength but is not the definition itself. Tensile stress is a concept that describes the internal forces within a material when it is subjected to stretching, but it does not characterize the maximum limit before deformation occurs. Compressive strength refers specifically to a material's ability to withstand axial loads that compress it, which is a different context from the general definition provided.

Thus, the correct term that encompasses the idea of maximum stress without permanent deformation is ultimate strength.