Alopecia areata: Patchy baldness that typically begins with rapid hair loss on discrete areas of the scalp and sometimes progresses to complete baldness and even loss of body hair. The characteristic diagnostic finding is short, broken hairs called 'exclamation point' hairs. Alopecia areata affects both males and females and, most often, children and young adults. It seems to be caused by an autoimmune mechanism, wherein the body's own immune system attacks the hair follicles and disrupts normal hair formation. Alopecia areata is sometimes associated with allergic disorders, thyroid disease, vitiligo, lupus, rheumatoid arthritis, ulcerative colitis, and other conditions, and some forms may be inherited. Hair can sometimes regrow within a year without treatment. The longer the period of time of hair loss, the less chance that the hair will regrow.