noun
Formal, often used in official or legal contexts such as obituaries, reports, or announcements. Not used in casual conversation.
事故の物故者は3名と発表された。
It was announced that there were three deceased in the accident.
物故者の名簿を作成する。
Compile a list of the deceased.
死亡者 simply means 'fatality' or 'person who died', without the formal, respectful nuance of 物故者.
物故 (ぶっこ, 'death') + 者 (しゃ, 'person'). 物故 is a Sino-Japanese compound meaning 'passing away', used in formal contexts.