areola

noun

1. The circular, darkly pigmented area surrounding a nipple; the areola mammae.

   “Working with the Personal Genome Project, she has collected measurements and images of the areolas of 150 people so far. One initial finding is that the diameter of the areola seems unrelated to the number of areola glands — the small bumps in the skin surrounding the nipple.”

2. (broadly)Any small circular area that is different from its immediate environment, such as the colored ring around the pupil of the eye (iris) or an inflamed region surrounding a pimple.
3. Any of the small spaces throughout areolar connective tissue.

   “The tubes or elongated spaces of which we have spoken, are not distended with any fluid, but are merely moistened in the same way as the areolas of ordinary areolar tissue.”

4. Any of the small spaces between fibres of the tissues of certain lichens.
5. Small patches, bearing the spines and glochids characteristic of the stems of cacti.

   “Every pulvillus or areola is studded with from six to eight spines... From these pulvilli, also called spiniferous areola, or very close to it, burst the young bud or flower. If the latter, it is then called the floriferous areola, and the point where the epidermis bursts is of a deep pink tinge.”