henotheism
Belief in or worship of one deity without denying the existence of other deities.
countable, uncountable
There is one kind of oneness which does not exclude the idea of plurality; there is another which does. […] If, therefore, an expression had been given to that primitive intuition of the Deity, which is the mainspring of all later religion, it would have been—'There is a God,' but not yet 'There is but "One God."' The latter form of faith, the belief in One God, is properly called monotheism, whereas the term of henotheism would best express the faith in a single god.
Where it comes from
From German Henotheismus, coined in the 19th century by German philosopher Friedrich Wilhelm Joseph von Schelling (1775–1854) from Ancient Greek ἕν (hén, stem of εἷς (heîs, “one”)) + German Theismus (“theism”, ultimately from Ancient Greek θεός (theós, “god”))).
Adapted from the Wiktionary entry henotheism (CC BY-SA 4.0).