Definitions for "Seasonal adjustment"
Adjusting a time series to measure and remove the regular seasonal components (e.g. the effect of Christmas on food retail sales).
Adjustment of time-series data to eliminate the effect of intra-year variations which tend to occur each year in approximately the same manner. Examples of such variations include school terms, holidays, weather patterns, etc.
A statistical method often used to adjust house price data to reflect the seasonal nature of house selling/buying. It is not unusual for seasonal adjustment to bias figures in unusual ways.