In anatomy, the term areola is used to describe any small circular area such as the dark skin surrounding the nipple. While it is most commonly used to describe the pigmented ring around the human nipple, it can also be used to describe other small circular areas such as the inflamed area surrounding a pimple.
There is a reason for the areola being a different color to the rest of the breast. The areola is roughly where the ducts of the mammary glands are. Careful inspection of a mature human nipple will reveal 15-20 small openings arranged radially around the tip of nipple (lactiferous ducts ) from where milk is released during lactation. Other small openings in the areola are sebaceous glands which may provide a little oily lubrication during lactation, otherwise known as Montgomery's glands (or glands of Montgomery).
Normal areolar color ranges from light pink to dark brown with races with lighter skin tones tending to have paler areolas and races with darker skin tones to have darker areolas. Areolar color changes to a darker shade during pregnancy. Some regression to the original color may occur thereafter but part of the change is permanent.
In Japan, where many women view dark areolas as a sign of aging, topical creams (such as this one) have been developed for the purpose of lightening (or "pinkening") the nipples. These products, which tend to ellicit a humorous reaction from westerners, are frequently included on Internet import sites (such as J-List.com).