阵亡（英语：Killed in Action，简称KIA）是军队当中广泛用来描述部队伤亡人员的军事用语，意指在战争中遭受敌军攻击或是友军误击而死亡的士兵。美国国防部声明，已经阵亡的军士主要是遭受敌军的攻击，无力反击时死亡，阵亡士兵来源不包括交通事故、恐怖主义及其他「非敌军」的原因，阵亡所意指可包括在前线作战的战斗队伍和陆海空的后援队伍中的士兵。

KIA代表一个士兵在战争中受到攻击而在到达医疗设施前死亡。而「受伤而死亡」（Die of Wounds，简称DOW）意指士兵生还至接受医学治疗，但是依然不治。北大西洋公约组织曾制定DWRIA取代DOW（Died of Wounds Received in Action），然而历史上军队与历史学家都使用前一种首字母缩略字（即KIA）。美军从二战以来都会将战争中失踪的军人在之后归类为失踪推定战死，如唐·费斯中校曾很短暂地被认定为失踪但一年后就被修改为战死尸体未寻获，在62年后因寻获遗体被修改分类为战死尸体确认（美国防部战俘及失踪人员办公室称为accounted for，未确认尸体者称为unaccounted for有时也会写成missing，如就朝鲜战争美军尚有八千左右missing，这并不意味着美军仍认定这些人活着，事实上民法都规定平民在失踪七年后即推定死亡的）。

Killed in action (KIA) is a casualty classification generally used by militaries to describe the deaths of their own combatants at the hands of hostile forces. The United States Department of Defense, for example, says that those declared KIA need not have fired their weapons but have been killed due to hostile attack. KIAs do not come from incidents such as accidental vehicle crashes and other "non-hostile" events or terrorism. KIA can be applied both to front-line combat troops and to naval, air and support troops. Someone who is killed in action during a particular event is denoted with a † (dagger) beside their name to signify their death in that event or events.