stretch mark

from The American Heritage® Dictionary of the English Language, 5th Edition.

• 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 usually as a result of pregnancy or weight gain.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a narrow band resulting from tension on the skin (as on abdominal skin after pregnancy)