In epidemiology, the excess deaths or excess mortality is a measure of the increase in the number 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").
Mortality displacement is the occurrence of deaths at an earlier time than they would have otherwise occurred, meaning the deaths are displaced from the future into the present, resulting in a changed life expectancy.
As opposed to number of registered fatalities of a certain death cause, such as a specific virus, a temporary excess mortality, and the mortality displacement, are measures that reflect many combined causes. The dominant reason may be events such as heat waves, cold spells, epidemics and pandemics (especially influenza pandemics), famine or war, and allows for estimates of the mortality caused by those events combined with other indirect health effects. Excess mortality is also studied for certain groups of people, such as elder, men, unemployed, etcetera.