Henotheism
Henotheism (I say HEE-no-thee-ih-zum, Greek hen "one" + theos "god") refers to the worship of a single deity while not denying other true gods exist and may be validly worshiped.
Henotheism is to be distinguished with monolatry, which rejects that other gods may be validly worshiped, and monotheism, which goes even further and says all other gods are false.