Brahmacharya is the only yama that is also the name of a stage of life. In the classical ashrama scheme the brahmacharin is the student: a boy invested with the sacred thread who moves into his teacher's household, serves the teacher, tends the fire, begs his food and learns his portion of the Veda, all under a strict rule of celibacy. The word itself says nothing about sex; it means conduct, charya, oriented to Brahman, and the sexual discipline was one element of a total dedication that included obedience, simplicity and study.
When the same word appears among Patanjali's yamas, both senses travel with it. For the renouncer traditions, yoga's natural constituency, brahmacharya meant what it meant for the student: complete celibacy, the containment of a force regarded as the body's most concentrated energy. Sutra 2.38 promises that on its perfection virya, vigor, is gained, and the physiological reading was taken seriously: seed conserved becomes ojas, vital radiance, the fuel of austerity and meditation. Householder tradition, which could hardly adopt celibacy wholesale, read the vow instead as fidelity and measure: sexuality confined to marriage and kept from becoming appetite. Modern yoga inherits both readings, and honest teachers present them as alternatives with different costs rather than pretending the tradition spoke with one voice.