Extremism means, literally "the quality or state of being extreme" or "advocacy of extreme measures or views".
Nowadays, the term is mostly used in a political or religious sense, for an ideology that is considered (by the speaker or by some implied shared social consensus) to be far outside the (acceptable) mainstream attitudes of society. But extremism can, for example, also be meant in an economic sense.
The term "extremism" is usually meant to be pejorative; that is, to express (strong) disapproval. However, it may also be meant in a more academic, purely descriptive, non-condemning sense.
Extremists are usually contrasted with centrists or moderates. For example, in contemporary discussions in Western countries of Islam or of Islamic political movements, the distinction between extremist (implying "bad") and moderate (implying "good") Muslims is typically 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.