Shibboleth

A shibboleth (/ˈʃɪbəlɛθ/ or /ˈʃɪbələθ/) is a word, sound, or custom that a person unfamiliar with its significance may not pronounce or perform correctly relative to those who are familiar with it. It is used to identify foreigners or those who do not belong to a particular class or group of people. It also refers to features of language, and particularly to a word or phrase whose pronunciation identifies a speaker as belonging to a particular group.