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Alopecia means loss of hair from the head or body. Alopecia includes baldness, a term generally reserved for pattern alopecia or androgenic alopecia. Compulsive pulling of hair (trichotillomania) can also produce hair loss, dubbed "traction alopecia". Hairstyling routines such as ponytails or braids may also induce hair loss, as can hair relaxer solutions, and hot hair irons. In some cases, alopecia is due to underlying medical conditions, such as iron deficiency.

Generally, patchy hair loss signifies alopecia areata. Alopecia areata typically presents with sudden hair loss causing patches to appear on the scalp or other areas of the body. If left untreated, or if the disease does not respond to treatment, complete baldness can result in the affected area, or alopecia totalis. When the entire body suffers from complete hair loss, it is alopecia universalis. It is similar to effects that occur with chemotherapy.