In Hinduism, most adherents venerate one or more deities but would regard these as manifestations of a single God or Ultimate Reality. The Ultimate Reality that is behind the universe and all the gods is called by different names, but most commonly brahman or brahma, sometimes capitalized as Brahman.
This neuter Sanskrit noun should not be confused with the creator god Brahma (a masculine noun) or the priestly class of brahmans/brahmins. It literally translates as "growth" or "expansion." (Oxford Dictionary of World Religions)
In the earliest Vedas, especially the Atharva Veda, Brahman is the mysterious force behind magic. In the Rig Veda, Ultimate Reality is referred to as "the One." In the Purushasukta, it is "Purusha," and in the Upanishads it is called "Brahman," "the One," and several other names.
Academic attempts to define brahman include the following:
"The eternal, conscious, irreducible, infinite, omnipresent, spiritual source of the universe of finiteness and change." ("brahman," Encyclopedia Britannica)
"The one supreme, all-pervading Spirit; the impersonal Absolute, beyond attributes, which is the origin and support of the visible universe." (Oxford Dictionary of World Religions)
Most Hindus believe in this concept of brahman and regard the various gods and goddesses as manifestations of this unknowable reality.
Vedanta philosophy, which is based on the Upanishads, centers on the concept of Brahman. Some schools teach that brahman is one with atman, the essence or Self of all beings, and only knowledge of Brahman will bring liberation from the cycle of reincarnation.