In Hinduism, Brahman (ब्रह्मन् brahman) is "the unchanging reality amidst and beyond the world", which "cannot be exactly defined", but is Sat-cit-ānanda (being-consciousness-bliss) and the highest reality.
Brahman is conceived as personal (Saguna Brahman, with qualities), impersonal (Nirguna Brahman, without qualities) and/or Para Brahman, supreme, depending on the philosophical school.
The sages of the Upanishads teach that Brahman is the ultimate essence of material phenomena (including the original identity of the human self) that cannot be seen or heard but whose nature can be known through the development of self-knowledge (atma jnana). According to Advaita, a liberated human being (jivanmukta) has realised Brahman as his or her own true self.
Radhakrishnan, who is representative of the "Modern Hinduism", refers to Brahman as the Absolute or Godhead which is the Divine Ground of all being.
The Isha Upanishad says:
Auṃ - That supreme Brahman is infinite, and this conditioned Brahman is infinite. The infinite proceeds from infinite. If you subtract the infinite from the infinite, the infinite remains alone.
Read more about Brahman: Etymology, Semantics and Pronunciation, Brahman in Jainism, See Also