Sectarianism names both a sociological category and a pattern of intergroup hostility, and the two senses are worth separating before either is used.
The sociological vocabulary derives from Max Weber and Ernst Troeltsch, who distinguished the church, an inclusive body into which one is born, broadly coextensive with society, accommodating to the world and tolerant of the imperfect member, from the sect, a voluntary body one joins by conviction, demanding of its members, exclusive, and in tension with the surrounding society. H. Richard Niebuhr added the observation that sects tend to become churches across generations, as the children of converts are born into what their parents chose, which supplies both a mechanism and a life cycle. Bryan Wilson later refined the typology by classifying sects according to their response to the world, conversionist, revolutionist, introversionist, manipulationist, thaumaturgical, reformist and utopian, which handles the diversity considerably better than a single axis.
The terminology has serious problems. "Sect" and its relatives carry pejorative freight in ordinary usage, and the French and German cognates are more damning still, which makes the scholarly term hard to use neutrally in public. The category was developed on Christian material and transfers awkwardly to traditions organised differently. And the church-sect distinction presupposes a religiously unified society against which a sect stands out, which describes early modern Europe far better than a plural modern one. Many scholars now prefer "new religious movement" for recent groups, precisely to avoid the loading, though that term has been criticised in turn as a euphemism that flattens real differences between benign and harmful groups.
In the second sense, sectarianism names hostility between religious communities, and the word is generally used where the division falls within a single tradition, Catholic and Protestant in Northern Ireland, Sunni and Shia across parts of the Middle East and South Asia. Scholarship on both cases converges on a common conclusion: such conflicts are rarely about doctrine, however consistently they are described in doctrinal terms by participants and observers alike. Communal identity, historical grievance, control of the state and of resources, and the deliberate political mobilisation of difference do most of the explanatory work, and the theological content usually functions as a marker of the boundary rather than as the reason for it. This is not an argument that religion is irrelevant, religious identity is what the boundary is drawn on, and religious institutions can inflame or restrain, but it is a strong argument against explaining such conflicts by reading the creeds of the parties.