doer; person of action
Refers to someone who actively takes initiative and gets things done, rather than just thinking or talking.
He's not just talk; he's a real doer.
A leader who is a person of action drives the team forward.
活動家 is often used for activists in social or political movements, whereas 行動家 is a broader term for anyone who acts decisively.
Compound of 行動 (action) and 家 (person who does or specializes in something).