Stretch marks or striae (singular stria), as they are called in dermatology, are a form of scarring on the skin with an off-color hue. They are caused by tearing of the dermis, and over time can diminish but not disappear completely.

Stretch marks are often the result of the rapid stretching of the skin associated with rapid growth (common in puberty) or weight gain (e.g. pregnancy or muscle building) or in some cases, severe pulling force on skin that overcomes the dermis's elasticity. Stretch marks may also be influenced by hormonal changes associated with puberty, pregnancy, muscle building, hormone replacement therapy for transsexuals, etc. Medical terminology for these kinds of markings includes striae atrophicae, vergetures, stria distensae, striae cutis distensae, striae gravidarum (in cases where it is caused by pregnancy), lineae atrophicae, striae distensae, linea albicante, or simply striae. (Via Wikipedia)