the eternal spirit or soul, called the Atman.

Brahman is a Vedic Sanskrit word, and it is conceptualized in Hinduism,as the "creative principle which lies realized in the whole world". Brahman is a key concept found in the Vedas, and it is extensively discussed in the early Upanishads.
Hindus believe that there is one true reality, the supreme spirit, called Brahman. Brahman 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.