The Collaborative International Dictionary of English v.0.48:

mayor \may"or\, n. [OE. maire, F. maire, fr. L. major greater,
   higher, nobler, compar. of magnus great; cf. Sp. mayor. See
   Major, and cf. Merino.]
   The chief magistrate of a city or borough; the chief
   administrative officer of a municipal corporation. In some
   American cities there is a city court of which the major is
   chief judge. The post is usually elective, its holder chosen
   by the electorate of the entire city.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mayor
    n 1: the head of a city government [syn: mayor, city
         manager]