Polytheism is opposed to monotheism, since there is not one God, but many Gods.  In other words, there is not one transcendent ultimate God, but a series of many Gods.  Thus, many ancient religions, like the Greeks and Romans, had a variety of Gods for all occasions.  The Israelites were continually fighting against the variety of Baal Gods in Canaan.  Many ancient native religions also had a variety of Gods to answer their many questions.