an Osman Turkish word meaning Governor. The governors of the various provinces of North Africa under Ottoman rule were known as Beys, e.g. the Bey of Tunis. The area ruled from Algiers, however, was divided into three Beyliks (provincial governments) with Beys at Oran, Constantine, and Médéa, all of whom served under the Dey of Algiers. The Dey (the Turkish word for a maternal uncle), chosen by the members of the Turkish military garrison from among their number, was nominally the viceroy of the Sultan, but was in reality the absolute ruler over the city of Algiers and its three subordinate Beyliks. Mustafà should therefore properly have been called Dey, not Bey. There actually was a Dey by that name who ruled from 1798 - 1805, not long before the libretto was written.
Bey is originally a Turkishhttp://www.m-w.com/dictionary/bey Merriam-Webster Online - Bey word for "chieftain," traditionally applied to the leaders of small tribal groups. In historical accounts, many Turkish, other Turkic and Persian leaders are titled Bey, Beg or Beigh. They are all the same word with the simple meaning of "leader."