'Trinitarianism' originates from Latin, specifically the word 'Trinitas', where 'trini-' meant 'three' and 'Trinitas' signified 'the state of being three'; the suffix '-ism' comes via Greek/Latin '-ismos', meaning 'belief, system'.

'Trinitas' gave rise to Medieval Latin forms such as 'Trinitarius' (Trinitarian) and Late Latin/Medieval usages like 'Trinitarianismus' or French 'trinitarianisme', which were borrowed into English as 'Trinitarianism'.

Initially it referred broadly to 'the state or condition of being three' (in Latin usage), but over time it evolved specifically into the theological sense 'the doctrine that God is three persons in one essence'.