Which skin condition is characterized by well-defined patches of hair loss?

Alopecia areata is characterized by well-defined patches of hair loss, often appearing suddenly and affecting discrete areas of the scalp or other body regions. This autoimmune condition occurs when the immune system mistakenly attacks hair follicles, leading to hair loss in specific spots. The borders of the patches are typically clear and well-defined, which helps differentiate this condition from others that may cause more diffuse or gradual thinning of hair.

In contrast, conditions like telogen effluvium result in diffuse thinning rather than distinct patches, typically occurring due to stress, hormonal changes, or nutritional deficiencies. Androgenetic alopecia, commonly known as male or female pattern baldness, leads to a gradual thinning across the scalp rather than isolated patches. Traction alopecia results from continuous pulling or tension on the hair, often due to hairstyles, and presents similarly as thinning or loss in specific areas but not as well-defined patches.