The Collaborative International Dictionary of English v.0.48:

Magna Carta \Mag"na Car"ta\, Magna Charta \Mag"na Char"ta\ [L.,
   great charter.]
   1. The great Charter, so called, obtained by the English
      barons from King John, A. D. 1215. This name is also given
      to the charter granted to the people of England in the
      ninth year of Henry III., and confirmed by Edward I.
      [1913 Webster]

   2. Hence, a fundamental constitution which guaranties rights
      and privileges.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Magna Carta
    n 1: the royal charter of political rights given to rebellious
         English barons by King John in 1215 [syn: Magna Carta,
         Magna Charta, The Great Charter]