Trichotillomania (TTM) (often called "trich") is an impulse control disorder. It causes a person to habitually pull out their hair. People with trichotillomania may pull hair from anywhere on the body. Sometimes people with trich end up having large areas on the body without hair.
Trichotillomania is listed in the Diagnostic and Statistical Manual of Mental Disorders (DSM). The DSM is written by the American Psychiatric Association. It lists every condition which is officially called a mental illness. The DSM puts trich in the "impulse control disorder" category. However, there are questions about what category trich should be in. At different times, it may seem to be a habit, an addiction, a tic disorder, or an obsessive-compulsive disorder.