An activist shareholder is one who uses an equity stake in a corporation not simply as an item within a portfolio but as an opportunity to redefine and redirect the management of that corporation.

Shareholder activism can take any of several forms: proxy battles, publicity campaigns, litigation, even in the right circumstances more-or-less amiable negotiations with management.

For example, in September 2003, hedge fund manager Lawrence J. Goldstein created a new fund specifically for the purpose of using such activism to increase the value of targeted corporations' equity. The resulting entity is known as the Santa Monica Partners Opportunity Fund LP.