A PCB leakage is a type of short, sometimes referred to as a high-resistance short. It is a partial connection between two or more nets that exhibits a resistance value between the limit defined for an acceptable continuity and an acceptable isolation.
A leakage path on a board can create metal migration from one trace to another. This happens while the board is in operation, to build a hard short. Sources are ionic contamination (deposits of metal salts) and moisture.