An island is a piece of land which is entirely surrounded by water for the majority of the day - if a link to the mainland is dry except at high tide then it's a peninsula, if it's only dry at low tide then it's still an island. Note that islands do not have to be seabound, they can also occur in rivers and lakes and be created by digging of canals - although an island surrounded only by canals is liable to be dismissed as something with a ditch as opposed to a true island.
A collection of islands is normally called an archipelago.
An interesting variant - relatively rare in real life but slightly more common in fiction - is the floating island which is not anchored to the bed of whatever body of water it floats in and so drifts about. Natural floating islands are normally composed of buoyant rock such as pumice or tufa but the Aztecs used artificial ones built on reed mats for agricultural purposes.
Another trope - this one a lot more common in fiction than in real life - is that of the island that rises up out of the sea and then vanishes again due to geological activity. When this happens in real life, it's usually a volcanic island that is destroyed again by a later erruption but in fiction it can be caused by bubbles of gas, plate tectonics, meddling deities or magical mishaps.
List of Noteworthy Islands
Real World Islands
Game and Story Use
- Islands are great for horror scenario, as they increase the sense of isolation and cut off possible escape routes.
- The floating island is an interesting plot device, but should not be overused.
- Artificial islands can be made by dumping dirt, but an interesting variant tech might be using advanced biotechnology or biothaumaturgy to grow them from coral or geoengineering / elemental magic / geomancy to create volcanic islands or artificially elevate part of the sea bed.