'homoousios' originates from Ancient Greek, specifically the word 'ὁμοούσιος' (homoousios), where 'ὁμός' (homo-) meant 'same' and 'οὐσία' (ousia) meant 'being' or 'substance'.

'homoousios' passed into Late Latin and ecclesiastical Latin as 'homoousios' and from theological Latin entered English usage in discussions of Trinitarian doctrine (via church councils and theological texts).

Initially it expressed the general idea 'of the same essence/substance'; over time it became specialized in Christian theology to denote the Son's consubstantiality with the Father in Trinitarian doctrine.