A schism (/ˈsɪzəm/ SIZ-əm, /ˈskɪzəm/, SKIZ-əm or, less commonly, /ˈʃɪzəm/ SHIZ-əm) is a division between people, usually belonging to an organization, movement, or religious denomination. The word is most frequently applied to a split in what had previously been a single religious body, such as the Great East–West Schism or the Western Schism. It is also used of a split within a non-religious organization or movement or, more broadly, of a separation between two or more people, be it brothers, friends, lovers, etc.

A schismatic is a person who creates or incites schism in an organization or who is a member of a splinter group. Schismatic as an adjective means pertaining to a schism or schisms, or to those ideas, policies, etc. that are thought to lead towards or promote schism.

In religion, the charge of schism is distinguished from that of heresy, since the offence of schism concerns not differences of belief or doctrine but promotion of, or the state of division, especially among groups with differing pastoral jurisdictions and authority. However, schisms frequently involve mutual accusations of heresy, and also that of the Great Apostasy. In Roman Catholic teaching, every heresy is a schism, while there may be some schisms free of the added guilt of heresy. Liberal Protestantism, however, has often preferred heresy over schism. Presbyterian scholar James I. McCord (quoted with approval by the Episcopalian Bishop of Virginia, Peter Lee) drew a distinction between them, teaching: "If you must make a choice between heresy and schism, always choose heresy. As a schismatic, you have torn and divided the body of Christ. Choose heresy every time."

The word schism comes from the Greek word σχίσμα, Greek transliteration: schisma which means "cleft, division".^{[citation needed]}

In Buddhism, the first schism was set up by Devadatta, during Buddha's life. This schism lasted only a short time. Later (after Buddha's death), the early Buddhist schools came into being, but were not schismatic,^{[citation needed]} only focusing on different interpretations for the same monastic community. In the old texts, 18 or 20 early schools are mentioned. Later, there were the Mahayana and Vajrayana movements, which can be regarded as being schismatic in origin. Each school has various subgroups, which often are schismatic in origin. For example, in Thai Theravadin Buddhism there are two groups (Mahanikaya and Dhammayut), of which the Dhammayut has its origin partly in the Mahanikaya, and is the new and schismatic group. Both Mahanikaya and Dhammayut have many subgroups, which usually do not have schismatic origins, but came into being in a natural way, through the popularity of a (leader) monk. Tibetan Buddhism has seen schisms in the past, of which most were healed, although the Drukpa school centred in Bhutan perhaps remains in a state of schism (since 1616) from the other Tibetan schools.^{[citation needed]}

The words schism and schismatic are used to denote splits within a church, denomination or religious body. In this context, "schismatic", as a noun, denotes a person who creates or incites schism in a church or a person who is a member of a splinter Church; as an adjective, "schismatic" refers to ideas and activities that are thought to lead to or constitute schism, and ultimately to departure from what the user of the word considers to be the true Christian Church. These words have been used to denote both the phenomenon of Christian group-splintering in general, and certain significant historical splits in particular.

One can make a distinction between heresy and schism. Heresy is rejection of a doctrine that a Church considered to be essential. Schism is a rejection of communion with the authorities of a Church, and not every break of communion is necessarily about doctrine, as is clear from examples such as the Western Schism and the breaking of the communion that existed between Patriarch Bartholomew I of Constantinople and Archbishop Christodoulos of Athens in 2004. However, when for any reason people withdraw from communion, two distinct ecclesiastical entities may result, each of which, or at least some members thereof, may then accuse the other(s) of heresy.

In Roman Catholic Church canon law, an act of schism, like an act of apostasy or heresy, automatically brings the penalty of excommunication on the individual who commits it. As stated in canon 1312 §1 1° of the 1983 Code of Canon Law, this penalty is intended to be medicinal, so as to lead to restoration of unity. Roman Catholic theology considers formal schismatics to be outside the Church, understanding by "formal schismatics" "persons who, knowing the true nature of the Church, have personally and deliberately committed the sin of schism". The situation, for instance, of those who have been brought up from childhood within a group not in full communion with Rome, but who have orthodox faith, is different: these are considered to be imperfectly, though not fully, related to the Church. This nuanced view applies especially to the Churches of Eastern Christianity, more particularly still to the Eastern Orthodox Church. While they don't possess "full communion" (communio in sacris) with the Catholic Church, they are still considered much more linked to it than the Protestant ecclesial communities, which have markedly different theological beliefs and rejected the concept of apostolic succession (with the exception of the Anglicans, which, however, are viewed by the Catholic Church as not having a valid priesthood).