But there's more to understanding a polymer's mechanical properties than merely knowing how strong it is. All strength tells us is how much stress is needed to break something. It doesn't tell us anything about what happens to our sample while we're trying to break it. That's where it pays to study the elongation behavior of a polymer sample. Elongation is a type of deformation.
Deformation is simply a change in shape that anything undergoes under stress.
When we're talking about tensile stress, the sample deforms by become stretching, becoming longer. We call this elongation, of course. There are a number of things we measure related to elongation. Which is most important depends on the type of material one is studying. Two important things we measure are ultimate elongation and elastic elongation Ultimate elongation is important for any kind of material.