A spiritual atom located in the left ventricle of the heart. This atom is also called the Master Atom and is directly related with the Christ.
"As we have already stated in our book entitled “The Perfect Matrimony or The Door to Enter into Initiation," the heart of the Sun is constructed like the heart of our human organism. Just as Seven Hierarchs exist in the Sun, who direct the seven cosmic rays, seven brains exist in our heart which belong to the Seven Great Cosmic Hierarchies. The sun has an atomic central nucleus, which is the Atom Nous, and this atom is the dwelling of Brahma within us. This atom is the first vital center which functions in the fetus and also the last atom which stops living in our organism. This atom contains the mind, life, energy and will power of the human being. It has an opal-like luminous aura which irradiates and shines." - Samael Aun Weor, The Revolution of Beelzebub.
For more information, read The Dayspring of Youth by M.