Extremism is a term used to describe the actions or ideologies of individuals or groups outside the perceived political center of a society; or otherwise claimed to violate common moral standards. The term is invariably, or almost invariably, used pejoratively. Extremism is usually contrasted with moderation, and extremists with moderates. (For example, in contemporary discussions in Western countries of Islam, or of Islamic political movements, it is common for there to be a heavy stress on the distinction between extremist and moderate Muslims).

Political agendas perceived as extremist often include those from the far left or far right as well as fundamentalism or, as a more general term, fanaticism.

Radicalism or extremism?

The terms extremism or extremist are almost always exonymic — i.e. applied by others to a group rather than by a group labeling itself. Rather than labeling themselves extremist, those labeled as such might describe themselves as, for example, political radicals. There is no political party that calls itself "right-wing extremist" or "left-wing extremist", and there is no sect of any religion that calls itself "extremist" or which calls its doctrine "extremism".

The term "extremist" is used to describe groups and individuals who have become radicalized, in some way, even though the term radical originally meant to go to the root of a (social) problem. The term "radical" is one not normally regarded as pejorative (except perhaps in the United States of America) and, unlike "extremist" is sometimes used by groups in their description of themselves.

The term "extremist" is often used with reference to those who use or advocate "violence" against the will of "society at large", but it is also used by some to describe those who advocate or use "violence" to enforce the will of the social body, such as a government or majority constituency. Those described as "extremist" would in general not accept that what they practice or advocate constitutes "violence" and would instead speak in terms of "acts of resistance" or "militant action" or "the use of force." The word "violence" cannot be regarded as "value-neutral". Ideology and "methodology" often become inextricably linked under the single term "extremism".

The notion that there is a philosophy which can be described as "extremism" is considered by some to be suspect. Within sociology, several academics who track (and are critical of) "extreme right-wing" groups have objected to the term "extremist", which was popularized by "centrist" sociologists in the 1960s and 1970s. As Jerome Himmelstein states the case: "At best this characterization tells us nothing substantive about the people it labels; at worst it paints a false picture." (Himmelstein, p. 7). The act of labeling a person, group or action as "extremist" is sometimes claimed to be a technique to further a political goal — especially by governments seeking to defend the status quo, or "political centrists". In any event, the term "extremist" — like the word "violence" — cannot be regarded as "value-neutral".