天主 is the Chinese term specifically used in Catholicism for God, introduced by Jesuit missionaries in the 16th century. It literally means 'Heavenly Lord' and is distinct from 上帝 (used in Protestant contexts) and 神 (a generic term for deity). In Catholic discourse, 天主 carries the theological weight of the triune God and is used in prayers, scripture, and church teachings. You'll want to use 天主 when referring to the God of Catholicism; for other monotheistic contexts, 上帝 or 神 may be more appropriate.