Brahmacharya is a divine word. It is the sum and substance of Yoga. Brahmacharya is the Achara or conduct by which you attain or reach Brahman (God). It is life in the Absolute. It is movement towards God or the Atman (Self).
Brahmacharya is absolute freedom from sexual thoughts and desires. It is the vow of celibacy. It is control of all the senses in thought, word and deed.
Brahmacharya is not mere bachelorhood. There should be strict abstinence not merely from sexual intercourse but also from auto-erotic manifestations, from masturbation, from homosexual acts and from all perverse sexual practices. It must further involve a permanent abstention from indulgence in erotic imagination and voluptuous reverie.
In a narrow sense, Brahmacharya is celibacy. In a broad sense, it is absolute control of all the senses. The door of Nirvana (liberation) or perfection is complete Brahmacharya.
Celibacy is to a Yogi what electricity is to an electric bulb. Without celibacy no spiritual progress is possible. It is a potent weapon and shield to wage war against the internal evil forces of lust, anger and greed. It serves as a gateway for the bliss beyond, and opens the door of liberation. It contributes perennial joy and uninterrupted bliss. It is the only key to open the Sushumna (the chief among astral tubes in the human body running inside the spinal column) and awaken the Kundalini (the primordial cosmic energy located in the individual).
There cannot be any language without words. You cannot draw a picture without a canvas or a wall. You cannot write anything without paper. Even so, you cannot have health and spiritual life without celibacy.
An established celibate will not feel any difference in touching the opposite sex, a piece of paper, a block of wood, or a piece of stone. A true celibate only can cultivate Bhakti (devotion). A true celibate only can practice Yoga. A true celibate only can acquire jnana (wisdom).
Brahmacharya is meant both for men and women. Bhishma, Hanuman, Lakshmana, Jesus, Mirabai, Sulabha and Gargi were all celibates.