Refers to death caused by an accident, disaster, or other unintended event, as opposed to natural death or murder. Often used in news reports and formal contexts.
There were casualties in the accident.
Casualties from the earthquake were unavoidable.
Compound of 人 (ひと, 'person') and 死に (しに, continuative form of 死ぬ, 'to die'). The reading ひとじに is a rendaku form.