Who is Brahman in Hinduism?

Hindus believe that there is one true god, the supreme spirit, called Brahman. Brahman has many forms, pervades the whole universe, and is symbolised by the sacred syllable Om (or Aum). Most Hindus believe that Brahman is present in every person as the eternal spirit or soul, called the atman

This is all i know

Brahma (ब्रह्म) (nominative singular), brahman (stem) (neuter gender) means the concept of the transcendent and immanent ultimate reality, Supreme Cosmic Spirit in Hinduism. The concept is central to Hindu philosophy, especially Vedanta; this is discussed below