The Upanishads, Sanskrit texts from the Vedic era, define brahman as satyam jnanam anantam brahma. Satyam means "that which never changes," jnanam means "knowledge," and anantam means "infinity."
Brahman is the essence of all that can be experienced but not seen, including the soul and the truth. It is also the cosmic soul. Every living being on earth comes from this cosmic soul and, as such, there is no spiritual distinction between people around the world. Brahman is the source of all reality and the grounding sustenance of all reality.
Paul Jakob Deussen, a Sanskrit scholar from Germany who was a friend of Hindu monk Swami Vivekananda, describes brahman as being the "creative principle" that is manifested in the whole world.
Yoga, as a spiritual discipline, helps the practitioner to understand and experience brahman. Yoga helps one master the senses and ultimately the mind, thereby bringing the Self closer to brahman, the spiritual source.
Brahman does not refer to the Hindu god Brahma, the Hindu god of creation. Nor does it refer to Brahmin, a class that is a part of the caste system as described in the Upanishads.