Seasonality looks at whether a currency pair has historically tended to move in a particular direction during a specific time of year — a particular month, or even a specific week — averaged across many past years. The idea is that some recurring calendar-linked flows (fiscal year-ends, commodity harvest cycles, year-end portfolio rebalancing) can create patterns that show up faintly in long-run historical averages.
The honest caveat: seasonality is a historical tendency, not a rule, and it's the weakest standalone signal of anything covered in this glossary — any single year can (and often does) break the pattern entirely if a strong enough fundamental driver is in play at the same time. It's best treated as one minor input layered alongside everything else, never a reason on its own.