a thin line or wrinkle that forms in the skin, esp. of the abdomen, hips, or thighs, after the dermis has been stretched excessively in a short period of time due to pregnancy, obesity, etc.
See stretch mark in American Heritage Dictionary 4
stretch mark
noun
A shiny line on the skin of the abdomen, breasts, thighs, or buttocks that is often lighter than the surrounding skin and is caused by the stretching and weakening of elastic tissues as a result of pregnancy or obesity, for example.