In monotheistic belief systems, God is usually viewed as the supreme being, creator, and principal object of faith. In polytheistic belief systems, a god is "a spirit or being believed to have created, or for controlling some part of the universe or life, for which such a deity is often worshipped". Belief in the existence of at least one deity, who may interact with the world, is called theism.

Conceptions of God vary considerably. Many notable theologians and philosophers have developed arguments for and against the existence of God. Atheism rejects the belief in any deity. Agnosticism is the belief that the existence of God is unknown or unknowable. Some theists view knowledge concerning God as derived from faith. God is often conceived as the greatest entity in existence. God is often believed to be the cause of all things and so is seen as the creator, sustainer, and ruler of the universe. God is often thought of as incorporeal and independent of the material creation, which was initially called pantheism, although church theologians, in attacking pantheism, described pantheism as the belief that God is the material universe itself. God is sometimes seen as omnibenevolent, while deism holds that God is not involved with humanity apart from creation.

Some traditions attach spiritual significance to maintaining some form of relationship with God, often involving acts such as worship and prayer, and see God as the source of all moral obligation. God is sometimes described without reference to gender, while others use terminology that is gender-specific. God is referred to by different names depending on the language and cultural tradition, sometimes with different titles of God used in reference to God's various attributes.

The earliest written form of the Germanic word God comes from the 6th-century Codex Argenteus, containing a Gothic translation of the Bible. The English word itself is derived from the Proto-Germanic *ǥuđan. The reconstructed Proto-Indo-European form *ǵhu-tó-m was probably based on the root *ǵhau(ə)-, which meant either 'to call' or 'to invoke'. The Germanic words for God were originally neuter, but during the process of the Christianization of the Germanic peoples from their indigenous Germanic paganism, the words became a masculine syntactic form. In English, capitalization is used when the word is used as a proper noun, as well as for other names by which a god is known. Consequently, the capitalized form of god is not used for multiple gods or when used to refer to the generic idea of a deity.

The English word God and its counterparts in other languages are normally used for any and all conceptions and, in spite of significant differences between religions, the term remains an English translation common to all.

The Hebrew word for 'god' is El, which also as a proper noun referred to the chief deity in ancient Semitic religions. In the Hebrew Bible, God is also given a personal name, Yahweh, in contrast to the genetic name, and in origin possibly the name of an Edomite or Midianite deity who was adopted into ancient Israelite religion. In many English translations of the Bible, Yahweh is translated as "the LORD" with "Lord" in all caps. Jah or Yah is an abbreviation of Jahweh/Yahweh, and often sees usage by Jews and Christians in the interjection "Hallelujah", meaning 'praise Jah', which is used to give God glory. In Judaism, some of the Hebrew titles of God are considered holy names.

Allāh (Arabic: الله) is the Arabic term with no plural used by Muslims and Arabic-speaking Christians and Jews meaning 'the God', while ʾilāh (إِلَٰه, plural `āliha آلِهَة) is the term used for a deity or a god in general. Muslims also use a multitude of other titles for God.

In Hinduism, Brahman is often considered a monistic concept of God. God may also be given a proper name in monotheistic currents of Hinduism which emphasize the personal nature of God, with early references to his name as Krishna-Vasudeva in Bhagavata or later Vishnu and Hari. Sang Hyang Widhi Wasa is the term used in Balinese Hinduism.