The Collaborative International Dictionary of English v.0.48:

Lateran \Lat"er*an\, n.
   The church and palace of St. John Lateran, the church being
   the cathedral church of Rome, and the highest in rank of all
   churches in the Catholic world.
   [1913 Webster]

   Note: The name is said to have been derived from that of the
         Laterani family, who possessed a palace on or near the
         spot where the church now stands. In this church
         several ecclesiastical councils, hence called Lateran
         councils, have been held.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Lateran
    n 1: the site in Rome containing the church of Rome and the
         Lateran Palace