'homoousianism' originates from Greek, specifically the word 'homoousios' plus the English suffix '-ism', where 'homo-' meant 'same' and 'ousia' meant 'essence, substance'.

'homoousios' (Greek) was used in early Church theology and Late Latin texts; from these theological and ecclesiastical uses the adjective 'homoousian' and the noun 'homoousianism' entered English usage in discussions of Christology from the Reformation period onward.

Initially the Greek term meant 'of the same substance' (a philosophical/ontological phrase); over time it became the technical theological label for the doctrine that the Son is consubstantial with the Father.