Merriam-Webster Collegiate Dictionary - trinitarian

 adjective  Date: 1628  1. capitalized of or relating to the Trinity, the doctrine of the Trinity, or adherents to that doctrine  2. having three parts or aspects ; threefold TRINITARIAN  noun  Date: 1628  1. a member of a religious teaching and nursing order for men founded in France in 1198 by John of Matha and Philip of Valois  2. one who subscribes to the doctrine of the Trinity  • ~ism noun