'antitrinitarian' originates from the prefix 'anti-' (from Greek anti, meaning 'against') combined with 'trinitarian' (from Latin 'Trinitas', meaning 'the state of being three or the Trinity').

'trinitarian' derives from Latin 'Trinitas' ( Medieval Latin trinitarius ), and the compound form 'antitrinitarian' developed in post-Reformation English to describe those opposing the doctrine of the Trinity; it was used in the 16th–17th centuries in theological disputes and later stabilized into modern English usage.

Initially used specifically in theological and doctrinal disputes to denote opposition to the Trinity, the term has retained that meaning and continues to denote either people or beliefs rejecting Trinitarian doctrine.