the main land mass of a country or continent; as distinguished from an island or peninsula
"The traveler flew from the mainland city to visit her family on a distant island."
The continent; the principal land, as distinguished from islands or a peninsula.
"The mainland is connected to the larger continent by a narrow strip of water that separates it from several small islands."
In plain English: The mainland is the large, continuous piece of land that connects to most other countries but excludes nearby islands.
"Many tourists fly from their island home to visit friends on the mainland for the weekend."
Usage: Use mainland to refer specifically to large continental masses like North America or Asia when contrasting them with nearby island nations such as Japan or Great Britain. Avoid using it for any arbitrary piece of dry land if you simply mean "land" without the geographic distinction between continents and islands.
The largest island in the Orkney Islands council area, to the north of Scotland.
"Mainland is the largest island in the Orkney archipelago, situated just north of the Scottish mainland."
The word mainland comes from the Middle English phrase mayne londe, which literally means "great land." It entered modern usage to describe a large piece of continuous land that is not an island.