Definition of 'Announcement Effect'
The impact on markets from the news that a change will occur at some future date. It can be used as a general term for the reaction to any development that affects trading, such as a change in dividend policy or a stock split. It is most often used, however, to describe investor reactions to changes in monetary policy, such as a hike or cut in a key interest rate level.
Also known as a "signal effect."
Investopedia explains 'Announcement Effect'
Stock traders eagerly await the announcement of changes in Federal Reserve policy, and stock volumes are notably higher on so-called Fed days; depending on the investment environment, volatility may be substantially heightened as well. Researchers have also found that trading on the day preceding announcements of Fed policy is relatively calm.