The Collaborative International Dictionary of English v.0.48:

Mainland \Main"land`\, n.
   The continent; the principal land; -- opposed to island, or
   peninsula. --Dryden.
   [1913 Webster]

         After the two wayfarers had crossed from the peninsula
         to the mainland.                         --Hawthorne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mainland
    n 1: the main land mass of a country or continent; as
         distinguished from an island or peninsula