We use this word in two primary ways.
1. Each vertebra of the spinal column can be referred to as a chamber.
2. In all ancient spiritual traditions, there are three levels of help and instruction. They have been known by many names, but generally can be defined as (1) public instruction (exoteric), (2) private instruction (mesoteric), and (3) secret instruction (esoteric). First Chamber is the public, introductory teaching. Second Chamber is for those who have committed themselves to chastity and the death of the ego; Second Chamber is kept secret as it is a sacred and profound trust. Third Chamber is an even more advanced level of help, which is also kept hermetically sealed. These Three Chambers reflect the universal nature of all true initiatic colleges: in Egypt the initiates of each level were called (1) Apprentice, (2) Journeyman or Craftsman, and (3) Master. In Tibet, the same system is preserved.
It is important to note that Samael Aun Weor emphasized the necessity for the Three Chamber system and any school that does not maintain this structure is lacking the connection to the Logos Samael. Only those institutions who maintain the Chambers can be said to be true Gnostic Schools, wherever they may be.
"We are therefore working, my dear brothers and sisters, to initiate the Era of Aquarius. We are working in order to save what is possible, meaning, those who allow themselves to be saved. This is why it is necessary that we shape our Gnostic Movements and that we organize them each time better; that we establish the Three Chambers. Our Gnostic Movements must have exactly Three Chambers. Each Lumisial must have Three Chambers for the instruction of our students. Our Gnostic Centers receive a name in a very pure language that flows like a river of gold that runs under the thick jungle of the sun; that name is LUMISIALS." - Samael Aun Weor, from a lecture entitled The Final Catastrophe