Selective mutism (SM) is a complex anxiety disorder when a person who are fully capable of talking and can understand language cannot speak in certain situations or to certain people - that is, they cannot speak in public. Selective mutism usually co-exists with shyness or social anxiety disorder, but can also exist with other conditions where socialisation is impaired such as Autism.
People with this disorder may remain silent even if the consequences of being silent are shame, social ostracism or even punishment.