Mainland is defined as "relating to or forming the main part of a country or continent, not including the islands around it [regardless of status under territorial jurisdiction by an entity]." The term is often politically, economically and/or demographically more significant than politically associated remote territories, such as exclaves or oceanic islands situated outside the continental shelf.

In geography, "mainland" can denote the continental (i.e. non-insular) part of any polity or the main island within an island nation. In geopolitics, "mainland" is sometimes used interchangeably with terms like metropole as an antonym to overseas territories. In the sense of "heartland", mainland is the opposite of periphery. In some language a separate concept of "mainland" is missing and is replaced with a "continental portion".

The term is relative: in Tasmania, continental Australia is the mainland, while to residents of Flinders Island, the main island of Tasmania is also "the mainland", although the geological Australian continent includes all the former plus the island of New Guinea and all the smaller islands (e.g. the Torres Strait Islands) in between.