General term for a person actively campaigning for social or political change. Often interchangeable with 活動家, but アクティビスト can sound more modern or international.
See also: 活動家
She is famous as an environmental activist.
activist shareholder
Finance-specific term for a shareholder who uses their equity stake to pressure a company's management. Often seen in business news.
そのアクティビストは経営陣に改革を要求した。
The activist shareholder demanded reforms from the management.