The Collaborative International Dictionary of English v.0.48:

Date line \Date line\
   The hypothetical line on the surface of the earth fixed by
   international or general agreement as a boundary on one side
   of which the same day shall have a different name and date in
   the calendar from its name and date on the other side. Also
   called International Date Line.
   [Webster 1913 Suppl. +PJC]

   Note: Speaking generally, the date line coincides with the
         meridian 180[deg] from Greenwich. It deflects between
         north latitudes 80[deg] and 45[deg], so that all Asia
         lies to the west, all North America, including the
         Aleutian Islands, to the east of the line; and between
         south latitudes 12[deg] and 56[deg], so that Chatham
         Island and the Tonga group lie to the west of it. A
         vessel crossing this line to the westward sets the date
         forward by one day, as from Sunday to Monday. A vessel
         crossing the line to the eastward sets the date back by
         one day, as from Monday to Sunday. Hawaii has the same
         day name as San Francisco; Manila, the same day name as
         Australia, and this is one day later than the day of
         Hawaii. Thus when it is Monday May 1st at San Francisco
         it is Tuesday may 2d at Manila.
         [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

International Date Line
    n 1: an imaginary line on the surface of the earth following
         (approximately) the 180th meridian [syn: dateline, date
         line, International Date Line]