A pawn island is one or a group of pawns separated by a file from pawns of the same side
The pawns at the edges of islands are not necessarily isolated or hanging, but they are limited in the support they can get without making weakness. So, having fewer pawn islands in the endgame is an advantage.
The term comes from Capablanca: in his game against Shipley, he enters a King and Pawn endgame with three pawn islands facing four; in his game against Kostic, he plays a Rook endgame with three Pawn islands against four
They are not easy to move and cannot easily be both protected, so, one or both may be a weakness (Fischer-Petrosian, Debbage-Lane, Matison-Nimzowitsch, Jepps-Regis)
However, if the capturing pawn moves towards the centre, as it should, there maybe compensating advantages of central control, a half-open file and perhaps the Bishop pair (Bronstein-Najdorf, Bhend-Hort)
* Danger signs - General things to watch out for:
1. leaving the king without sufficient support from other pieces
2. weakness of the eighth rank
3. entering a lasting pin
4. poisoned pawns
5. placing pieces without escape routes -- Amazia AVNI