(1):

(n.) A word which was made the criterion by which to distinguish the Ephraimites from the Gileadites. The Ephraimites, not being able to pronounce sh, called the word sibboleth. See Judges xii.

(2):

(n.) Hence, the criterion, test, or watchword of a party; a party cry or pet phrase.

(3):

(n.) Also in an extended sense.