Seasonality

Seasonality is a repeating, predictable pattern in a metric tied to time cycles — hour of day, day of week, month of year.

Seasonality is not noise but structure; recognizing it is the difference between needless panic and a right decision. User behavior changes with the hour, the day of week and the season: weekend traffic differs from midweek, purchases jump on occasions, and a news site peaks at specific hours. The main danger is casting an ordinary seasonal dip as a problem, or reading an occasion-driven jump as durable success. So the correct comparison is 'week over prior week' or 'this month last year', not 'today versus yesterday'. Seasonality affects analysis too: any experiment or measurement must cover at least one full cycle, or it sees only a slice of the pattern. Separating seasonality from the real trend is a precondition for any reliable forecast.