Bey (Ottoman Turkish: بك/Bey, Arabic: بيه / Beye, Persian: بگ / Beg or Beyg) is a Turkish title for chieftain, traditionally applied to the leaders (for men) of small tribal groups. The feminine equivalent title was Begum. The regions or provinces where "beys" ruled or which they administered were called beylik, roughly meaning "khanate", "emirate" or "principality" in the first case, "province" or "governorate" in the second (the equivalent of duchy in other parts of Europe). Today, the word is still used informally as a social title for men (somewhat like the English word "mister" and the French word monsieur, which literally means "my lord"). Unlike "mister" however, it follows the name and is used generally with first names and not with last names.
The word entered English from Turkish bey, itself derived from Old Turkic beg, which - in the form bäg - has been mentioned as early as in the Orkhon inscriptions (8th century AD) and is usually translated as "tribal leader". The dialect variations bäk, bek, bey, biy, bi, and pig all derive from the Old Turkic form. The actual origin of the word is still disputed, though it is mostly agreed that it was a loan-word, in Old Turkic. This Turkic word is usually considered a borrowing from an Iranian language. However, German Turkologist Gerhard Doerfer assessed the derivation from Iranian as superficially attractive but quite uncertain, and pointed out the possibility that the word may be genuinely Turkic. Two principal etymologies have been proposed by scholars:
1. the Middle Persian title bag (also baγ or βaγ,Old Iranian baga; cf. Sanskrit भग / bhaga) meaning "lord" and "master".Peter Golden derives the word via Sogdian bġy from the same Iranian root. All Middle Iranian languages retain forms derived from baga- in the sense "god": Middle Persian bay (plur. bayān, baʾān), Parthian baγ, Bactrian bago, Sogdian βγ-, and were used as honorific titles of kings and other men of high rank in the meaning of "lord". The Iranian bāy (through connection with Old Indian noun bhāgá "possessions, lot") gave Turkish word bai (rich), whence Mongol name Bayan (rich).