In the context of rail electrification, a "diesel island" is a relatively short section of non-electrified railway which is surrounded by or connected to electrified lines. They are called diesel "islands" because generally they can only be used by diesel trains. Such sections pose a disadvantage in terms of operation, because electric trains from the surrounding lines cannot run on the isolated diesel section.
United Kingdom
References
Diesel island Wikipedia(Text) CC BY-SA