The common definition:

1. abstention from sexual relations.
2. abstention by vow from marriage: the celibacy of priests.
3. the state of being unmarried.

Etymology:

• 1663, formed in Eng. from L. cælibatus "state of being unmarried," from cælebs "unmarried," from PIE base *kaiwelo- "alone" + lib(h)s- "living." Celibate first recorded 1829. - Online Etymology Dictionary

In the esoteric tradition, it is known that there are two forms of celibacy.

1. Lunar
2. Solar

Solar is a positive type of sexual life, inwhich an individual refrains from sexual union and masturbation, replacing them with spiritual exercises that transmute the sexual matter. Unfortunately, many modern groups have abandoned these types of exercies, so that their celibates no longer have the tools to manage their sexual energy effectively, thus they are practicing the lunar type of celibacy. What modern people usually call "celibacy" is actually a state of sexual repression, called in esotericism "lunar brahmacharya," whose results speak for themselves, with the infamous perversities of priests, monks, and nuns who abuse children and each other, by means of homosexuality and masturbation. In addition, those who repress their sexual forces suffer mental imbalances, leaning towards fanaticism and crulety. The Inquisition and various types of religious wars stand as clear examples of results of lunar brahmacharya.

The lunar type [of brahmacharya] is that absurd sexual abstinence that serves only to produce filthy, nocturnal sexual pollutions with all of its fatal consequences. - Samael Aun Weor

1. during adolesence (before marriage)
2. in religious traditions, while passing through the levels of the teaching that are preparation for marriage or sexual cooperation (tantra, maithuna, karmamudra)
3. voluntarily after the death of one's spouse or because of damage to the sexual organs
4. after the spiritual benefits of marriage have been completed (i.e. the Second Birth) whether in the present or past existences, like St. Paul or the Dalai Lama

Celibacy or sexual abstention - properly utilized and guided - is an important aspect of spiritual life. A single person can develop a great deal of self-knowledge, as Paramahansa Yogananda did. However, it should be noted that he refused the higher teachings of sexual cooperation, and thus could not advance further (the masters of his Kriya Yoga lineage practiced with their wives, but with Yogananda, that tradition ended).

Marriage - the sacrament of sexual union - has always been a sacred institution whose spiritual importance was emphasized in all religions. In all ancient traditions, the highest spiritual leaders were always married, even if secretly. Many famous "celibates" practiced sexual cooperation in secret. A key example is Sri Ramakrishna who practiced sacred sexuality devotedly with his partner.

According to Indian tradition, Ramakrishna had Sarada Devi sit on the throne of the Divine Mother within the temple.  At the same time, he began to sing the hymn to Devi Kundalini with the ancestral ritual ceremony, which culminated in the famous Shorashi Puja, the adoration of woman. He and she achieved Samadhi during the Maithuna...  - Samael Aun Weor, The Mystery of the Golden Blossom (1972)

One cannot reach the heights of the path alone. In the beginning of spiritual life, many need a time of preparation for sexual cooperation. This is why brahmacharya or celibacy has long been instituted. Yet, through celibacy, one cannot master the sexual powers, which create life in the superior worlds.

There were also some hallucinated mystics who pleaded in favor of an absurd type of chastity [without transmutation], which some religious sects preach, but do not practice.  They ignore that Nature is in rebellion against that harmful abstinence, which causes nocturnal pollutions and the decalcification of the organism through the urethra.  Sickness is the consequence.

Men were made for women and women for men, because Nature is wise in its designs.  What man and woman have to learn is to enjoy themselves without hurting each other, and the answer for this is Sexual Magic. During the act of love, the Gnostic couple transmutes their semen into atomic energy by restraining the sexual act.  This energy then rises upwards towards their head, through certain spermatic canals. - Samael Aun Weor, The Revolution of Beelzebub (1950)

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