In epidemiology, the excess deaths or excess mortality is a measure of the increase in the number of deaths during a time period and/or in a certain group, as compared to the expected value or statistical trend during a reference period (typically of five years) or in a reference population. It may typically be measured in percentage points, or in number of deaths per time unit.
A short period of excess mortality that is followed by a compensating period of mortality deficit (i.e., fewer deaths than expected, because those people have died at a younger age) is quite common, and is also known as "harvesting". Mortality deficit in a particular time period can be caused by deaths displaced to an earlier time (due to harvesting by an event in the past) or deaths displaced to a future time (due to lives being saved, also called "avoided mortality").