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]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "mayor":
   alderman, archon, bailie, burghermaster, burgomaster,
   cabinet member, cabinet minister, chancellor, city councilman,
   city father, city manager, commissar, commissioner, councillor,
   councilman, councilwoman, county commissioner, county supervisor,
   elder, headman, induna, legislator, lord mayor, magistrate, maire,
   minister, minister of state, portreeve, reeve, secretary,
   secretary of state, selectman, supervisor, syndic, undersecretary,
   warden



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MAYOR, officer. The chief or executive magistrate of a city who bears this
title.
     2. It is generally his duty to cause the laws of the city to be
enforced, and to superintend inferior officers, such as constables, watchmen
and the like. But the power and authority which mayors possess being given
to them by local regulations, vary in different places.