shibboleth.

word of the day**: shibboleth \SHIB-uh-luth\
1 : catchword, slogan
2 : a widely held belief or truism
3 : a custom or usage regarded as distinctive of a particular group

I found the following especially interesting because I just read this account in Judges not too long ago:

The Bible's Book of Judges (12:4-6) tells the story of the Ephraimites, who, after they were routed by the Gileadite army, tried to retreat by sneaking across a ford of the Jordan River that was held by their enemy. The Gileadites, wary of the ploy, asked every soldier who tried to cross if he was an Ephraimite. When the soldier said "no," he was asked to say "shibboleth" (which means "stream" in Hebrew). Gileadites pronounced the word "shibboleth," but Ephramites said "sibboleth." Anyone who left out the initial "sh" was killed on the spot. When English speakers first borrowed "shibboleth," they used it to mean "test phrase," but it has acquired additional meanings since that time. (www.m-w.com)

Aren't words fascinating? I have nothing else to add.

**This really was the word of the day according to Merriam-Webster Online.