Fanaticism is a belief or behavior involving uncritical zeal, particularly for an extreme religious or political cause or in some cases sports, or with an obsessive enthusiasm for a pastime or hobby. Philosopher George Santayana defines fanaticism as "redoubling your effort when you have forgotten your aim"; according to Winston Churchill, "A fanatic is one who can't change his mind and won't change the subject". By either description the fanatic displays very strict standards and little tolerance for contrary ideas or opinions.

In his book Crazy Talk, Stupid Talk, Neil Postman states that "the key to all fanatical beliefs is that they are self-confirming....(some beliefs are) fanatical not because they are 'false', but because they are expressed in such a way that they can never be shown to be false."

The behavior of a fan with overwhelming enthusiasm for a given subject is differentiated from the behavior of a fanatic by the fanatic's violation of prevailing social norms. Though the fan's behavior may be judged as odd or eccentric, it does not violate such norms. A fanatic differs from a crank, in that a crank is defined as a person who holds a position or opinion which is so far from the norm as to appear ludicrous and/or probably wrong, such as a belief in a Flat Earth. In contrast, the subject of the fanatic's obsession may be "normal", such as an interest in religion or politics, except that the scale of the person's involvement, devotion, or obsession with the activity or cause is abnormal or disproportionate.