According to Hewitt's book, "Understanding Terrorism in America";  extremism can be defined statistically, stylistically, and in terms of societal attitudes. Statistical extremism is when attitudes in general or of a particular issue can be arranged in continuum with most of the population in the middle, then those at the extremes, left or right are extremists in a purely statistical sense.

Extremism is more an issue of style than content. Style extremism is one that hampers our understanding of important issues, clouds our discourse with diatribe, defamation, self-righteousness, fanaticism, and hatred, and impairs our ability to make intelligent, well-informed choices.

Characteristics of extremism include: character assassination by questioning motives, qualifications, past associations and mental health rather than dealing with the facts or issues raised; name calling and labeling that diverts attention from arguments; irresponsible sweeping generalizations; Inadequate proof for assertions; advocacy of double standards; tendency to view opponents and critics as essentially evil; tendency to see world in terms of absolutes of good and evil; tendency to identify themselves in terms of who their enemies are; tendency to use argument by intimidation; and emphasis on emotional responses and, correspondingly, a de-emphasis on reasoning and logical analysis.

Social extremism is the representation of the larger population's legitimate interests that the extremists are able to secure. It is a mistake to imagine extremists as isolated actors. Frequently, extremists are fringes that represent what the larger community is unable (or unwilling) to represent. The dichotomy is particularly relevant when violence is used. Violent extremists may be a fringe not supported by the population because of the use of violence, but are in tune with the larger group's desire to obtain the same political goals.

References:

Hewitt, Christopher. Understanding Terrorism in America: From the Klan to Al Qaeda.

Southern Poverty Law Center - Fighting Hate, Teaching Tolerance, Seeking Justice
http://www.splcenter.org/