A shibboleth (// or //) is a custom, principle, or belief distinguishing a particular class or group of people, especially a long-standing one regarded as outmoded or no longer important. It usually refers to features of language, and particularly to a word whose pronunciation identifies its speaker as being a member or not a member of a particular group.
"... a word whose pronunciation identifies its speaker as being a member or not a member of a particular group."
Wikipedia's section Notable Shibboleths is nothing less than chilling. It gives several references of words that, when incorrectly pronounced, not only marked the speaker as a member or not a member of a group, but resulted in their summary execution.
This page describes a shibboleth as "a kind of linguistic password."
From the Book of Judges in the Old Testament, King James Bible:
5 And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay;
6 Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand.
Thanks to http://www.biblegateway.com/ for the above passage.