In materials science and engineering, elongation is a measure of a material's ability to deform under tensile stress. The most common definition is the permanent extension of a test sample after it has fractured in a tensile test. The subscripts 50 and 80 indicate the gauge length expressed in millimeters, while A5 means that the gauge length is for a so-called proportional specimen. Nowadays this is only called A.
A5, A50, and A80 elongation values are commonly reported in material specifications, quality control documents and research papers.