noun
the deceased; deaths; persons killed
Refers to people who have died, especially in the context of accidents, disasters, or statistics. Often used in news reports.
事故の死亡者は3名と報告された。
The number of deaths in the accident was reported as three.
地震による死亡者数はまだ確定していない。
The death toll from the earthquake has not yet been confirmed.
死亡 is the noun for 'death' itself, while 死亡者 specifically refers to the people who died.
Compound of 死亡 (death) and 者 (person). The reading is standard on'yomi.