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An island is a landmass that is surrounded by water and smaller than a continent. Islands occur in oceans, seas, lakes, and rivers and range from a few square meters to thousands of square kilometers. The term covers both inhabited and uninhabited places, and it is often contrasted with continents and mainlands. Distinctions among related terms—such as islets or skerries—reflect differences in size and permanence.

Islands form through various geological processes. Oceanic islands arise from volcanic activity or from the eruption of seamounts that reach the surface. Continental islands are pieces of continental crust separated by sea level rise or tectonic movement. Coral atolls develop from reef growth around sunken volcanic islands, while barrier islands and sandbanks result from sediment deposition. In some regions, sea-level rise or glacial rebound has created tidal or drowned landscapes where land is intermittently connected to the mainland.

Ecology and human use: Islands often host distinct ecosystems because isolation reduces immigration of species. They can harbor high levels of endemism and specialized habitats, such as mangroves, reefs, or cloud forests. Humans use islands for fisheries, farming, tourism, and habitation; many islands are inhabited while others are protected as nature reserves. Islands face challenges from invasive species, habitat loss, and climate change, especially sea-level rise and extreme weather.

Examples: Archipelagos such as the Philippines, Indonesia, and the Caribbean include thousands of islands. In northern Europe, the Baltic Sea contains extensive archipelagoes, including the Stockholm archipelago, consisting of thousands of islands and islets. The study of islands intersects geology, biology, and regional planning.