A person who attempts to use his or her rights as a shareholder of a publicly-traded corporation to bring about social change. Some of the issues most often addressed by shareholder activists are related to the environment, investments in politically sensitive parts of the world and workers' rights .
The term can also refer to investors who believe that a company's management is doing a bad job and who attempt to gain control of the company and replace management for the good of the shareholders.
Taobiz explains Shareholder Activist
Shareholder activism is a way in which shareholders can influence a corporation's behavior by exercising their rights as owners. Although shareholders don't run a company, there are ways for them to influence the board of directors and management. These can range from dialogue with management to voice their concerns about a particular issue to formal proposals that are voted on by all shareholders at a company's annual meetings.