shibboleth

From Dictionary.com/Word of the Day

shibboleth \SHIB-uh-lith; -leth\, noun:
1. A word or pronunciation that distinguishes a particular class or set of persons from another.
2. A word or saying identified with a group or cause; a slogan; a catchword.
3. A saying or belief identified with a particular group and usually regarded by outsiders as meaningless or untrue.
4. A custom, practice, behavior, etc. regarded as distinctive of a particular group.

Shibboleth is from Hebrew shibboleth, "stream, flood," from the use of this word in the Bible (Judges 12:4-6) as a test to distinguish Gileadites from Ephraimites, who could not say 'sh' but only 's' as in 'sibboleth'.