The primary similarity between polytheism and monotheism is the belief in at least one god, or divine being. Both belief systems are considered forms of theism.
Theism is the belief in one or more supreme beings. Theists view their respective god or gods as having a conscious mind and being the cause of all things in the universe. Both systems differ greatly from atheism, which is the belief in no god whatsoever.
Both words have Greek roots. Monotheism is derived from the Greek words "monos," meaning "one," and "theos," meaning "god." The "poly" in polytheism comes from the Greek word for "many."
Both polytheism and monotheism are practiced widely around the world. Christianity, Judaism and Islam are the three most widely practiced monotheistic religions. Hinduism and Mahayana Buddhism are widely practiced polytheistic religions. Adherents of both systems worship their god or gods through prayer and ceremony.
Some polytheistic religions share some elements of monotheistic belief, and vice versa. For example, Hindus worship many different gods. However, these gods are manifestations of a single divine entity named Brahman. Another example is that some interpret the Christian belief in the Holy Trinity as having polytheistic elements. The Holy Trinity consists of the Father (God), the Son (Jesus) and the Holy Spirit. These three distinct entities are considered divine within the Christian belief, which holds that the three elements are aspects of the single God.