Brahmacharya is a Sanskrit word, which means "control of the creative energy". Brahmacharya is one of the yamas and niyamas, the "dos and don'ts" of practicing any form of yoga.
Many Tantric traditions suggest that men refrain from ejaculating, not just during Tantric sex, but at all times. This is a way to meet the yogic requirement for brahmacharya or "control of the creative energy". In contrast, ascetic traditions interpret brahmacharya as a requirement for complete celibacy.
Physical orgasm is an important part of Tantric orgasm, particularly in the early years of practice. Physical orgasm is a separate physical process from ejaculation, even though in untrained men they usually occur together. Separating physical orgasm from ejaculation is an important part of a man's process of mastering the Tantric orgasm.