Definition of Eustathian
plural -s
1
[Eustathius of Sebaste †ab a.d.380 Semi-Arian bishop of Sebaste in Armenia + English -an, noun suffix]
:a follower of the Semi-Arian bishop Eustathius who established a monastic institute which was condemned by the Synod of Gangra in a.d.340
2
[Eustathius of Antioch †ab a.d.360 bishop of Antioch in Syria + English -an, noun suffix]
:one of an orthodox party whose protest against the deposition by an Arian synod of Eustathius, bishop of Antioch, led to a schism that lasted till a.d.413 
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