Time series that show regular patterns of movement within a year across years.
Seasonal lags are most often included as a lagged value one year before the prior value
For quarterly data, the fourth autocorrelation will not be statistically zero if there is quarterly seasonality. For monthly, the 12th, and so on.
To correct for seasonality, we can include an additional lagged term to capture the seasonality.
Xt = B0 + B1Xt-1 + B2Xt-4 +ei