The Collaborative International Dictionary of English v.0.48:

Sabbat \Sab"bat\, n. [See Sabbath.]
   In mediaeval demonology, the nocturnal assembly in which
   demons and sorcerers were thought to celebrate their orgies.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sabbat
    n 1: a midnight meeting of witches to practice witchcraft and
         sorcery; in the Middle Ages it was supposed to be a demonic
         orgy [syn: sabbat, witches' Sabbath]