Definition: Monotheism is a belief in one god. It's like henotheism, but in henotheism, one's focus is on one god, and the henotheist doesn't deny the existence of the other gods. Henotheism is applicable to Hinduism, Isis worship, and the worship of Julian the Apostate, among others. The Amarna period of Egyptian history, headed by Pharaoh Akhenaten and his Great Wife Nefertiti, is often described as monotheistic, but henotheism fits better since Akhenaten didn't deny the existence of the other gods.
Many consider Zoroastrianism the first monotheistic religion.