Sectarianism may, in the abstract, be characterized by dogmatism and inflexibility

Sectarianism refers (usually pejoratively) to a rigid adherence to a particular sect or party or religious denomination. It often implies discrimination, denunciation, or violence against those outside the sect. The term is most often used to refer to religious sectarianism, involving conflict between members of different religions or denominations of the same religion. It is also frequently used to refer to political sectarianism, generally on the part of a tight-knit political faction or party.
Sectarianism may, in the abstract, be characterized by dogmatism and inflexibility; sentimental or axiomatic adherence to an idea, belief or tradition; and idealism that provides a sense of continuity, orientation, and certainty. As a pejorative term, accusations of sectarianism may sometimes be used to demonize an opposing group.

The ideological underpinnings of attitudes and behaviours labelled as sectarian are extraordinarily varied. Members of a religious group may feel that their own salvation requires aggressively seeking converts from other groups; adherents of a given faction may believe that for the achievement of their own political or religious project their opponents must be purged. Sometimes a group feeling itself to be under economic or political pressure will attack members of another group thought to be responsible for its own decline. At other times, sectarianism may be the expression of a group's nationalistic or cultural ambitions, or cynically exploited to serve an individual demagogue's ambition.
In all cases, there is a real or felt opposition between 'Us' and 'Them', between insiders and outsiders. Sectarianism may take the form of hatred and fear of an outside sect or group. In such cases, sectarianism does not require a strong sense of religious belief, as much as a sense of group belonging.

A sectarian conflict usually refers to violent conflict along religious and political lines such as the conflicts between Catholics and Protestants in Northern Ireland. It may also refer to general philosophical or political conflict between different schools of thought such as that between Shia and Sunni Muslims. Non-sectarians espouse that free association and tolerance of different beliefs are the cornerstone to successful peaceful human interaction.
In the United States, the word sectarian is also traditionally used non-pejoratively to describe institutions of higher education - colleges and universities - affiliated with religions or religious denominations, in comparison to institutions not so affiliated.