In general terms, a recession can be any period where theres a decline in economic activity. For a period to qualify as a real recession, however, it typically has to be marked by sustained negative growth in GDP along with an increase in unemployment.
Example: In February of 2020, due to the rise of COVID cases and impending lockdowns, the United States officially entered a recession following a 5% drop in GDP and an increase in the unemployment rate up to nearly 15%.