In Vedanta, Nirguna Brahman, or Parabrahman is the Supreme Godhead (the Unmanifest Absolute).   The Atman, or Paramatman is the Supreme Self.  These two are actually one and the same

The Atman - Brahman state is the of the nature of infinite Being, infinite Consciousness, infinite Bliss (Sachchidananda).  It is unitary, perfect, eternal, unchanging, and encompasses all opposites and all possibilities within itself.  It is the Supreme Self (Paramatman) or One Consciousness within all beings; that Reality or I-ness or Awareness which remains in all possible states of consciousness and without which one would not exist.  It is the Source and Essence of all beings; for everything that exists is in its essence THAT.  Yet because it is beyond all opposites it is also beyond human conceptual understanding, and so can only be described imperfectly.

Links

Atman - wikipedia

The Divine Self - A Collection of Yogic Aphorisms - compiled and edited by Robert E. Wilkinson (archived page - original no longer on-line)