An "island" at Ephemerisle is a collection of one or more anchored boats, platforms, and/or whatever else floats that's the home of at least one person during the gathering. Generally they have a theme.
Islands are largely independent and autonomous, though they may be interconnected to varying degrees (by walkways, shared art projects, etc).
Islands are encouraged to experiment with their own governance systems (though currently benevolent dictatorships seem to be winning out.)
Every island is strongly encouraged to treat it's neighbors with respect and not doing anything (including anchoring too closely) which might put their neighbors in danger.
A collection of islands connected by one or more footbridges is an archipelago. The boundary between an island and archipelago is a fuzzy one, but generally speaking, islands which are part of an archipelago are largely independent of eachother, in terms of governance, social cohesion, and purpose, but collaborate together to create and maintain the archipelago infrastructure (anchors, lighting, and lines for the bridges).