Alopecia areata

Alopecia areata is a form of hair loss from areas of the body, usually from the scalp. Because it causes bald spots on the head, especially in the first stages, it is sometimes called spot baldness. In 1–2% of cases, the condition can spread to the entire scalp (Alopecia totalis) or to the entire epidermis (Alopecia universalis).
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Alopecia ar·e·a·ta n. Hair loss in circumscribed, noninflamed areas of the scalp, eyebrows and beard. Also called alopecia circumscripta. [more]Alopecia areata - Community and Resources

Noun
1. patchy baldness
(hypernym) alopecia

Patchy baldness (alopecia means baldness and areata means occurring in patches). The problem typically begins with patchy hair loss on the scalp and sometimes progresses to complete baldness and even loss of body hair. Although alopecia areata affects 2.5 million people in the United States alone, little is known about its underlying causes. Stress, the immune system, and several different genes may play a part in causing alopecia areata.