行动 (xíngdòng) is a broad word for 'action' or 'to act', often used for a planned operation, military move, or taking steps to achieve a goal. It is more formal and purposeful than 动作 (dòngzuò 'movement, motion'), which refers to physical gestures or body movements. Unlike 活动 (huódòng 'activity, event'), 行动 emphasizes decisive steps rather than general activities.