Monotheism is the belief in the existence of one deity, or in the oneness or uniqueness of God. It is a type of Theism, and is usually contrasted with Polytheism (the belief in multiple gods) and Atheism ( the absence of any belief in gods). The Abrahamic faiths (Judaism, Christianity and Islam), as well as Plato's concept of God, all affirm monotheism, and this is the usual conception debated within Western Philosophy of Religion.

The word "monotheism" is derived from the Greek ("monos" meaning "one" and "theos" meaning "god"), and the English term was first used by the English philosopher Henry More (1614 - 1687).

The earliest monotheistic religions can be traced back to the Aten cult in ancient Egypt, the Nasadiya Sukta from the Vedic period of India, and Ahura Mazda, the one uncreated Creator of Zoroastrianism. There are also monotheistic denominations within Hinduism, including Vedanta, Vaishnavism, Shaivism, Shaktism, and Smartism.

The Torah (or Hebrew Bible), which was created between the 13th Century and 4th Century B.C., is the source of Judaism, and in turn provided the basis for the Christian and Islamic religions (these three together being known as the Abrahamic faiths). Jews, Christians and Muslims would probably all agree that God is an eternally existent being that exists apart from space and time, who is the creator of the universe, and is omnipotent (all-powerful), omniscient (all-knowing), omnibenevolent (all-good or all-loving) and possibly omnipresent (all-present). The religions, however, differ in the details: Christians, for example, would further affirm that there are three aspects to God (the Father, the Son and the Holy Spirit).

More recently, Sikhism is a distinctly monotheistic faith that arose in northern India during the 16th and 17th Centuries, and the Baha'i faith, a religion founded in 19th Century Persia, has as it core teaching the one supernatural being, God, who created all existence.

Philosophical monotheism, and the associated concept of absolute good and evil, emerged in classical Greece, notably with Plato and the subsequent Neo-Platonists (who developed a kind of theistic monism in which the absolute is identified with the divine, either as an impersonal or a personal God).

• Exclusive Monotheism:
  The belief that there is only one deity, and that all other claimed deities are distinct from it and false. The Abrahamic religions, and the Hindu denomination of Vaishnavism (which regards the worship of anyone other than Vishnu as incorrect) are examples of Exclusive Monotheism.
• Inclusive monotheism:
  The belief that there is only one deity, and that all other claimed deities are just different names for it. The Hindu denomination of Smartism is an example of Inclusive Monotheism.
• Substance Monotheism:
  The belief (found in some indigenous African religions) that the many gods are just different forms of a single underlying substance.
• Pantheism:
  The belief in one God who is equivalent to Nature or the physical universe, or that everything is of an all-encompassing immanent abstract God.
• Panentheism:
  The belief (also known as Monistic Monotheism), similar to Pantheism, that the physical universe is joined to, or an integral part of, God, but stressing that God is greater than (rather than equivalent to) the universe.
• Deism:
  A form of monotheism in which it is believed that one God exists, but that this God does not intervene in the world, or interfere with human life and the laws of the universe. It posits a non-interventionist creator who permits the universe to run itself according to natural laws.
• Henotheism:
  The devotion to a single god while accepting the existence of other gods, and without denying that others can with equal truth worship different gods. It has been called "monotheism in principle and polytheism in fact".
• Monolatrism (or Monolatry):
  The belief in the existence of many gods, but with the consistent worship of only one deity. Unlike Henotheism, Monolatrism asserts that there is only one god who is worthy of worship, though other gods are known to exist. This is really more Polytheism than Monotheism.
• Misotheism:
  The belief that a God exists, but is actually evil. The English word was coined by Thomas de Quincey in 1846. Strictly speaking, the term connotes an attitude of hatred towards God, rather than making a statement about His nature.
• Dystheism:
  The belief that a God exists, but is not wholly good, or possibly even evil (as opposed to eutheism, the belief that God exists and is wholly good). There are various examples of arguable dystheism in the Bible.