Phantasiasts (from Greek φαντασία, phantasia, appearance, phantasm) was a label applied to several distinct Christian heresies by their opponents in late antiquity.
In a letter read before the Council of Chalcedon (451), Pope Leo the Great castigates the phantasmatici Christiani (Christian phantasmatics) in a clear reference to the Eutychians.
[1] The use of the label Phantasiasts by both Dyophysites and moderate Miaphysites indicates the extreme nature of the position relative to orthodox theologies.
[3] In the middle of the sixth century, the term Phantasiasts was applied to the Aphthartodocetae, the followers of Julian of Halicarnassus, the theological foe of Severus of Antioch.
It was in this sense that Patriarch Sergius I of Antioch used the term in the late 550s when writing to the Miaphysite bishops of Persia about receiving back those who had lapsed into the "heresy of Julian the Phantasiast".