What is Seasonal adjustment?

A statistical method that removes from an economic time series the effects that recur every year at roughly the same time and with similar strength (for example, the effects of weather or holidays). The aim is to see whether the underlying trend, not the season, has changed. Special statistical models and software are used for the adjustment. In the adjusted series the trend and the business cycle are seen more clearly.

Source: Eurostat Glossary: Seasonal adjustment; BEA Glossary: seasonal adjustment