In chess, a pawn island is a group of like-coloured pawns on adjacent files. Which rank the pawns are on is irrelevant; for example, if the pawn structure looks like the diagram below, then we have three separate pawn islands, regardless of the fact that the leftmost pair are somewhat disconnected:

+---+---+---+---+---+---+---+---+
| P |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   | P |   |   | P |   |
+---+---+---+---+---+---+---+---+
|   | P |   | P |   | P |   | P |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+

Because pawns on adjacent files are capable of protecting one another, pawns with neighbours are stronger than those without; for this reason, increasing the number of pawn islands is generally considered to weaken the pawn structure.