Activism consists of intentional action to bring about social, political, economic, or environmental change. This action is in support of, or opposition to, one side of an often controversial argument. In contemporary use, "activism" tends to be a word associated with the actions and ideologies of those on the political left.

The word "activism" is used synonymously with protest or dissent, but activism can take a wide range of forms from writing letters to newspapers or politicians, political campaigning, economic activism such as boycotts or preferentially patronizing businesses, rallies, street marches, strikes, both work stoppages and hunger strikes, or even guerrilla tactics.

In some cases, activism has nothing to do with protest or confrontation. Some religious or vegetarian/vegan activists try to persuade people to change their behavior directly, rather than persuade governments to change laws. The cooperative movement seeks to build new institutions which conform to cooperative principles, and generally does not lobby or protest politically.