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.