I think of trading activism in the same way as consumer activism.
Consumer activism is a process by which activists seek to influence the way in which goods or services are produced or delivered. Kozinets and Handelman attempt to define the broad concept as any social movement that uses society's drive for consumption to the detriment of business interests. -- wikipedia: Consumer activism
In the same way that an activist consumer may boycott a product that is cheaper, better or even essential to them in order to apply pressure to a producer or a distributor to change their policies (for example using sweatshops, destroying palm trees, or lobbying for a specific politician), an activist trader may buy or sell stocks or other financial instruments in order to make an ideological point by driving out of buisness or causing significant financial damage to a company, an institution, or a fund that is considered to have negative social impact.
For example, the some individual traders kept buying $GME stock at any price, in hope of buncrupting hedge funds because they consider that these funds are bad for society. They knew that they may loose their small investment, but they did not care, as the expected profit for them was to bring down a hedge fund. 
Activist trading has become feasible by services like Robinhood that allow retail traders invest small amounts and social networks like reddit that allows the coordination and aggregation of these activists.
Much like activist consumers, activist traders do not act in their best financial interest.
This may be a new reality that shakes the fundamental assumption behind financial markets that all parties act in their best financial interest. Publicly traded companies, individual and institutional investors, brokers, funds and other parties like banks that are part of the financial system will have to take this into account.
 I'm not saying that all $GME traders acted with this motive in mind. I am refering to the ones that bought and hold $GME "to bring the f---- hedge funds down".
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