Sanskrit ओजस्. Literally, "power or force of life, vigor, splendor, strength, power, lustre, vitality, water, energy, light."
Ojas is the energy that results from positive, pure sexual transmutation (brahmacharya), achieved through very specific techniques.
"The Yogis claim that of all the energies that are in the human body the highest is what they call "Ojas". Now this Ojas is stored up in the brain, and the more Ojas is in a man's head, the more powerful he is, the more intellectual, the more spiritually strong. One man may speak beautiful language and beautiful thoughts, but they, do not impress people; another man speaks neither beautiful language nor beautiful thoughts, yet his words charm. Every movement of his is powerful. That is the power of Ojas. Now in every man there is more or less of this Ojas stored up. All the forces that are working in the body in their highest become Ojas. You must remember that it is only a question of transformation. The same force which is working outside as electricity or magnetism will become changed into inner force; the same forces that are working as muscular energy will be changed into Ojas. The Yogis say that that part of the human energy which is expressed as sex energy, in sexual thought, when checked and controlled, easily becomes changed into Ojas, and as the Muladhara guides these, the Yogi pays particular attention to that centre. He tries to take up all his sexual energy and convert it into Ojas. It is only the chaste man or woman who can make the Ojas rise and store it in the brain; that is why chastity has always been considered the highest virtue. A man feels that if he is unchaste, spirituality goes away, he loses mental vigour and moral stamina. That is why in all the religious orders in the world which have produced spiritual giants you will always find absolute chastity insisted upon. That is why the monks came into existence, giving up marriage. There must be perfect chastity in thought, word, and deed; without it the practice of Raja-Yoga is dangerous, and may lead to insanity. If people practice Raja-Yoga and at the same time lead an impure life, how can they expect to become Yogis?" —Swami Vivekananda, Raja Yoga
"Sublimation is not a matter of suppression or repression, but a positive, dynamic, conversion process. It is the process of controlling the sex energy, conserving it, then diverting it into higher channels, and finally, converting it into spiritual energy or Ojas Sakti. The material energy is changed into spiritual energy, just as heat is changed into light and electricity. Just as a chemical substance is sublimated or purified by raising the substance through heat into vapour which again is condensed into solid form, so also, the sexual energy is purified and changed into divine energy by spiritual Sadhana." - Swami Sivananda, Brahmacharya
"The Sanskrit term Ojas means “transmuted sexual energy,” a type of Christic force. The semen is transmuted by means of Maithuna into subtle vapors, and these in turn convert themselves into energies that bipolarize in order to rise through Ida and Pingala up to the brain. To be converted into Ojas (Christic force) the semen must become “cerebrated.” In order to be charged with Ojas (Christic force), the brain must become “semenized.”” - Samael Aun Weor