stretch mark

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Wiktionary

1. n. Any of a series of red, irregular stripes on the surface of the skin caused by rapid growth of the tissues lying just underneath. They usually appear as a result of pregnancy, puberty or obesity, but can also be caused by rapid muscle growth.

WordNet 3.0

1. n. a narrow band resulting from tension on the skin (as on abdominal skin after pregnancy)