A person or group holding a significant amount of shares in a public company with the aim of influencing the decisions and structure of the organisation. 
The best examples of the good that activists can do is when they develop theses on how to make the companies they target run differently and better before they take their positions. For example, Bill Ackman, founder of hedge fund Pershing Square, led a proxy fight against Canadian Pacific Railway, took control of the board of directors, threw the management out and doubled the share price for himself and other shareholders in the process. 
The sweeping victory was just the latest sign that restless shareholders are frustrated with stagnant share prices and increasingly willing to challenge the executives supposed to serve their interests.
During 2012’s febrile annual meeting season in North America and the UK, which became known as a “shareholder spring”, management at companies such as Citigroup and Barclays were rebuked over executive pay. Even Apple – now the most valuable company in history – was forced to change the way it elects board members.