Source : Webster's Revised Unabridged Dictionary (1913)

Sabbat \Sab"bat\, n. [See {Sabbath}.]
   In medi[ae]val demonology, the nocturnal assembly in which
   demons and sorcerers were thought to celebrate their orgies.

Source : WordNet®

sabbat
     n : a midnight meeting of witches to practice witchcraft and
         sorcery; in the Middle Ages it was supposed to be a
         demonic orgy [syn: {witches' Sabbath}]