suicidal action; death-defying act; action taken with readiness to die
Refers to an act undertaken with full acceptance that it may lead to one's death; often used in dramatic or historical contexts.
He charged into the enemy camp, prepared for a suicidal action.
That mission was truly a death-defying act.
自殺行為 means 'suicidal act' and is more direct about the act being self-destructive, while 決死行 emphasizes the resolve to face death in pursuit of a goal.
Compound of 決死 (けっし, 'determined to die') and 行 (こう, 'action' or 'going'). The exact historical derivation is uncertain, but the word likely emerged in military or dramatic contexts.