(Sanskrit आकाश; or akasa) "Space, sky, atmosphere, vacuity, ether, free or open space, subtle and ethereal fluid, heaven, god brahma." From akash, "to be visible, appear, shine, be brilliant."
"A subtle agent that penetrates and permeates the whole space." - Samael Aun Weor, The Perfect Matrimony
The most subtle level of matter. The solar light comes to earth and everything on earth transforms that light. Prana is the light from the Absolute. When it passes through the law of seven it becomes the Akasha. Akasha is further modified into the tattvas and the elements.
Akasha is the name of the first tattva, the primordial substance that inundates the entire space; all the other tattva (Fire-Tejas, Air-Vayu, Water-Apas and Earth-Prithvi) are modifications of these cosmic waters, all of them live and work within Akasha. All forms, thoughts, dreams and ideas of the universe live in Akasha. There is no living thing in the universe that can exist without Akasha or multiply its species without it. Akasha is the substance from which every other substance emerges; Akasha is that which every other tattva emerges from; Akasha is where everything is in potency, and Akasha is where everything is in activity. Thus, Akasa appears in various positions. Wherever there is any room for any substance, there is Akasha.
"By dint of His will, the Lord, the undecaying substratum or reality of the universe, gave the first impetus to Nature to shake off her state of primal equipoise and to be gradually and successfully evolved into those categories and elements which were necessary for the formation of the present universe. The first evolute is Akasa. Why should Akasa be the first evolute? Because, without space, nothing can exist. Prana acted on Akasa. There was Spandana or vibration. Wherever there is vibration, there must be motion. Motion is the quality of air. Therefore, air came out of Akasa. Motion produced heat. Therefore, fire was born of Vayu or air. When there is heat, water is produced. On a hot day, the body perspires. Hence, water was born of fire. Wherever there is water, there is food. Earth is Annam or food. Therefore, earth was born of water. The subtler the element, the more powerful it is. Water is more powerful than earth, because it is more subtle than earth. Water removes away earth. Fire is more powerful than water, because it is more subtle than water. Fire dries up all water. Air is more powerful than fire, because it is more subtle than fire. Air blows up fire. Ether or Akasa is more powerful than air, because it is more subtle than air. Air rests in Akasa. Akasa is the support for air. Air is born of Akasa, fire is born of air, water is born of fire, earth is born of water. During cosmic Pralaya, earth is reduced or involved into water, water into fire, fire into air, and air into Akasa." —Swami Sivananda
"According to the Hindu Philosophy the whole nature is composed of two principal substances. One of them is called the Akasa or ether and the other, Prana or energy. These two may be said to correspond to matter and force of the modern scientists. Everything in this universe that possesses form or that has material existence, is evolved out of this omnipresent and all-pervasive subtle substance ‘Akasa’. Gas, liquid and solid, the whole universe, consisting of our solar system and millions of huge systems like ours and in fact every kind of existence that may be brought under the word ‘created’, are the products of this one subtle and invisible Akasa and at the end of each cycle return to the starting point. In the same way, all the way of forces of nature that are known to man; gravitation, light, heat, electricity, magnetism all those that can be grouped under the generic name of ‘energy’, physical creation, nerve-currents, all such as are known as animal forces and thought and other intellectual forces also, may be said to be the manifestations of the cosmic Prana. From Prana, they spring into existence and in Prana, they finally subside. Every kind of force in this universe, physical or mental can be resolved into this original force. There can be nothing new except these two factors in some one of their forms. Conservation of matter and conservation of energy are the two fundamental laws of nature. While one teaches that the sum total of Akasa forming the universe, is constant, the other teaches that the sum total of energy that vibrates the universe, is also a constant quantity. At the end of each cycle the different manifestations of energy quiet down and become potential: so also the Akasa which becomes indistinguishable: but at the beginning of the next cycle the energies start up again and act on the Akasa so as to involve the various forms. Accordingly, when the Akasa changes and becomes gross or subtle, Prana also changes and becomes gross or subtle. As the human body is only a microcosm to a Yogi, his body composed of the nervous system and the internal organs of perception represent to him, the microcosmic Akasa, the nerve-currents and thought-currents, and the cosmic Prana. To understand the secrets of their workings and to control them is, therefore, to get the highest knowledge and the conquest of the universe." —Swami Sivananda