Extremism is "the quality or state of being extreme" or "the advocacy of extreme measures or views". The term is primarily used in a political or religious sense to refer to an ideology that is considered (by the speaker or by some implied shared social consensus) to be far outside the mainstream attitudes of society. It can also be used in an economic context. The term may be used pejoratively by opposing groups, but is also used in academic and journalistic circles in a purely descriptive and non-condemning sense.

Extremists' views are typically contrasted with those of moderates. In Western countries for example, in contemporary discourse on Islam or on Islamic political movements, the distinction between extremist and moderate Muslims is commonly stressed. Political agendas perceived as extremist often include those from the far-left politics or far-right politics, as well as radicalism, reactionism, fundamentalism, and fanaticism.