homoousian
One of those, in the 4th century, who accepted the Nicene Creed and maintained that the Son had the same essence or substance with the Father.
historical
The Greek word, which was chosen to express this mysterious resemblance, bears so close an affinity to the orthodox symbol, that the profane of every age have derided the furious contests which the difference of a single diphthong excited between the Homoousians and the Homoiousians.
Where it comes from
From Ancient Greek ὁμοούσιος (homooúsios) + -an, from ὁμός (homós, “same”) + οὐσία (ousía, “essence”).
Adapted from the Wiktionary entry homoousian (CC BY-SA 4.0).