The ability of hair to stretch and return to its original length without breaking is referred to as hair:

The ability of hair to stretch and return to its original length without breaking is indeed referred to as elasticity. Elasticity is a key characteristic of hair that indicates its flexibility and strength. When hair is exposed to moisture, it can swell and elongate, showcasing its elastic properties. A hair strand with good elasticity can withstand manipulation, such as styling and chemical treatments, without sustaining damage.

Moisture levels can affect elasticity, but they are not synonymous; rather, moisture contributes to elasticity. Density, on the other hand, refers to the number of hairs per square inch on the scalp, while structure relates to the overall shape and organization of hair strands and follicles. These concepts, while related to hair's characteristics, do not define the ability to stretch and retract, which is the essence of elasticity.