The Collaborative International Dictionary of English v.0.48:

Alcalde \Al*cal"de\, n. [Sp. alcalde, fr. Ar. al-q[=a]d[imac]
   judge, fr. qada to decide, judge. Hence, the cadi of the
   Turks. Cf. Cadi.]
   A magistrate or judge in Spain and in Spanish America, etc.
   --Prescott.
   [1913 Webster]

   Note: Sometimes confounded with Alcaid.
         [1913 Webster]

WordNet (r) 3.0 (2006):

alcalde
    n 1: a mayor or chief magistrate of a Spanish town