Extremism is a concept used to describe religious, social or political belief systems that exist substantially outside of belief systems more broadly accepted in society (i.e., "mainstream" beliefs). Extreme ideologies often seek radical changes in the nature of government, religion or society. Extremism can also be used to refer to the radical wings of broader movements, such as the anti-abortion movement or the environmental movement. Not every extremist movement is "bad," the abolitionist movement is one example of an extreme movement that had admirable goals, but most extremist movements exist outside of the mainstream because many of their views or tactics are objectionable.

This database provides an overview of many of the terms and individuals used by or associated with movements and groups that subscribe to and/or promote extremist or hateful ideologies.