A shibboleth is a unique practice which can be used to distinguish between different ethnic, social, cultural, and regional groups. Most commonly, people use this term in reference to language, talking about specific words and methods of pronunciation which are unique. However, a shibboleth can take a variety of forms, from a shared memory of a common experience like going to the same college to a specific mode of dress.
This term has its roots in the Bible. There's a story in the Book of Judges in which there is a battle between the tribe of Ephraim and the people of Gilead. The Gileadites are victorious, and they take control of the river Jordan to prevent the Ephraimites from crossing over. To tell the difference between native Gileadites and Ephraimites pretending to be from Gilead, anyone who wishes to cross the river is presented with a test phrase: the Hebrew word for “torrent.” The people of Gilead used a “sh” pronunciation, saying “shibboleth,” while the Ephraimites said “sibboleth,” lacking the soft “sh,” and were instantly identifiable.
Any number of examples of shibboleths can be found across the world. In the Arabic-speaking world, for example, Egyptians do not use the hard “J” sound common to other Arabic speakers, so they say many key words very differently. British speakers talk about “shed-yooling” something, while Americans “skej-ool.” People from some parts of the world refrain from eating tomatoes, viewing them with deep suspicion, while others relish them. In New York, you stand “on line,” while everywhere else in the English-speaking world, people stand “in line.” Mammal owners recognize each other by the common shibboleth of pet hair on garments, while people who have had the same surgery identify each other by their scars.
Historically, shibboleths have often been used as passphrases, just like in the Bible. Soldiers in the same unit, for example, might use an obscure piece of native slang as a shibboleth. Shibboleths can also play a more subtle social and cultural role. For example, people with certain accents often have access to higher social circles because they share shibboleths in common with people in those circles, while people with accents associated with the lower classes are excluded.
Customs are exaggerated in some circles to make people feel more connected to each other. Many colleges and universities, for example, have unique slang terms, phrases, and other customs which graduates can use to readily identify each other. Garments such as school ties may be shibboleths, acting like secret codes to convey information without explicitly stating it.