A continuous land mass which has a distinct identity, and which
is larger than a certain arbitrary size:
Antarctica
The list of "continents" is shaped by two forces, which sometimes act in
concert, and sometimes in opposition:
-
Physical geography resulting from geological processes. To a geologist, a "continent" is a mass of sial or continental crust, plus the sediments that have washed off of it. This includes any continental shelf in addition to the part above water.
The physical limits of most continents are fairly obvious; at least the bits
above water. If we use geological criteria exclusively, Europe and Asia
merge into one continent, Eurasia. The two subcontinents of India
and Arabia each have a tectonic identity far stronger
than Europe's. Finally, Indonesia emerges as an eighth continent, mostly drowned since the end of the last ice age. The
traditonal discrimination between Australia as a continent and Greenland
as an island is strengthened by the fact that Australia is on its own tectonic plate, and Greenland is not.
-
Cultural geography forces, especially the historical identities of continents
and political motivaitons for retaining those identities. The distinction
between Europe and Asia was made long before any geologists entered the
picture, when Europe meant Greece (well, Crete really) and Asia meant
Anatolia. Africa was a province of the Roman Empire, today's
Tunisia. Arabia Felix had its own identity distinct from
Asia.
As the size of the traditional continents expanded with knowledge
of the world, so did the size requirement for a "continent". The
fabulous Southern Continent was a device used by mapmakers to fill in an
empty space on the world map; the continent actually discovered, Antarctica,
was far different from the tropical paradise expected (except for that
bit to the north, Australia).