Nestorianism is a term used in Christian theology and Church history to refer to several mutually related but doctrinarily distinct sets of teachings. The first meaning of the term is related to the original teachings of Christian theologian Nestorius (d. c. 450 AD), who promoted specific doctrines in the fields of Christology and Mariology. The second meaning of the term is much wider, and relates to a set of later theological teachings, that were traditionally labeled as Nestorian, but differ from the teachings of Nestorius in origin, scope and terminology. The Oxford English Dictionary defines Nestorianism as "The doctrine of Nestorius, patriarch of Constantinople (appointed in 428), by which Christ is asserted to have had distinct human and divine persons."

Original Nestorianism is attested primarily by works of Nestorius, and also by other theological and historical sources that are related to his teachings in the fields of Mariology and Christology. His theology was influenced by teachings of Theodore of Mopsuestia (d. 428), the most prominent theologian of the Antiochian School. Nestorian Mariology rejects the title Theotokos ('God-bearer') for Mary, thus emphasizing distinction between divine and human aspects of the Incarnation. Nestorian Christology promotes the concept of a prosopic union of two natures (divine and human) in Jesus Christ, thus trying to avoid and replace the concept of a hypostatic union. This Christological position is defined as radical dyophysitism, and differs from orthodox dyophysitism, that was reaffirmed at the Council of Chalcedon (451). Such teachings brought Nestorius into conflict with other prominent church leaders, most notably Cyril of Alexandria, who issued 12 anathemas against him (430). Nestorius and his teachings were eventually condemned as heretical at the Council of Ephesus in 431, and again at the Council of Chalcedon in 451. His teachings were considered as heretical not only in Chalcedonian Christianity, but even more in Oriental Orthodoxy.

After the condemnation, some supporters of Nestorius, who were followers of the Antiochian School and the School of Edessa, relocated to the Sasanian Empire, where they were affiliated with the local Christian community, known as the Church of the East. During the period from 484 to 612, gradual development led to the creation of specific doctrinal views within the Church of the East. Evolution of those views was finalized by prominent East Syriac theologian Babai the Great (d. 628) who was using the specific Syriac term qnoma (ܩܢܘܡܐ) as a designation for dual (divine and human) substances within one prosopon (person or hypostasis) of Christ. Such views were officially adopted by the Church of the East at a council held in 612. Opponents of such views labeled them as "Nestorian" thus creating the practice of misnaming the Church of the East as Nestorian. For a long time, such labeling seemed appropriate, since Nestorius is officially venerated as a saint in the Church of the East. In modern religious studies, this label has been criticized as improper and misleading. As a consequence, the use of Nestorian label in scholarly literature, and also in the field of inter-denominational relations, is gradually being reduced to its primary meaning, focused on the original teachings of Nestorius.