An event study is an empirical research method used "to analyze the market reaction to firm specific and market-wide events using either returns or volume around the time when the event occurred" (Event Study by WRDS description). Examples of events you may wish to study include earnings announcements, M&A activity, or leadership turnover.
Event Study by WRDS
Event Study by WRDS is a tool you can use to do an event study. Use your own events or use the Capital IQ Key Developments. See also this video introduction.