The work of an activist investor is described in full in the entry Activist Investing, which covers the tactics, the history, notable campaigns and the criticism. This page gives a short definition of the person or firm behind it.
An activist investor is an individual or fund that buys a meaningful stake in a listed company in order to change how it is run, rather than simply waiting for the price to rise. Demands typically include a new strategy, cost cuts, the sale of a division, a larger buyback or seats on the board of directors. Many activists are hedge funds, and their tools range from private letters to public campaigns and proxy voting fights at the annual meeting.
"Activist investor" names the actor and "activist investing" names the activity, so one page covers both. The idea sits close to shareholder value debates, and the same pressure tactics now appear in crypto, where large token holders sometimes push a project's governance vote in a particular direction.