Dharmadhatu (Skt. dharmadhātu; Tib. ཆོས་ཀྱི་དབྱིངས་, chö ying, Wyl. chos kyi dbyings) — literally ‘the essence or expanse of phenomena’. All-encompassing space. Dharmadhatu can be synonymous with buddha nature.
Khenchen Thrangu Rinpoche writes:
- The word for space is དབྱིངས་, ying in Tibetan, dhātu in Sanskrit. […] The word space is used because the dharmadhatu is like the body or realm of empty space where different things, like clouds, birds, and airplanes can fly around without obstruction. This is because the nature of space is empty and nonexistent. Due to this quality of openness, things can occur. Likewise, dharmadhatu is the essence of things—empty and inconcrete—where all phenomena such as trees, houses, mountains, oneself, other beings, emotions, wisdom, and all experiences can occur openly." 
Tulku Urgyen Rinpoche says:
- The main image of dharmadhatu is that of space—the ‘space of all things’ within which all phenomena manifest, abide and dissolve back into. […] Dharmadhatu is the basic environment of all phenomena, whether they belong to samsara or nirvana. It encompasses whatever appears and exists, including the worlds and all beings.[…] The relationship between dharmadhatu, dharmakaya and [the wisdom of dharmadhatu] is like the relationship between a place, a person and the person’s mind. If there is no place, there is no environment for the person to exist in; and there is no person unless that person also has a mind dwelling in the body. In the same way, the main field or realm called dharmadhatu has the nature of dharmakaya. Dharmakaya has the quality of [the wisdom of dharmadhatu], which is like the mind aspect. […] "Dharmadhatu is adorned with dharmakaya, which is endowed with [the wisdom of dharmadhatu]." This is a brief but very profound statement, because ‘dharmadhatu’ also refers to sugatagarbha or buddha nature.
- basic space of phenomena
- realm of phenomena
- sphere of reality
- Thrangu Rinpoche, Buddha Nature (Bookpeople, 1996), page reference needed.
- Tulku Urgyen Rinpoche, As It Is, Vol. I (Boudhanath, Hong Kong & Esby: Rangjung Yeshe, 1999), pages 31-32.
- Tulku Urgyen Rinpoche, As It Is, Vol. I (Boudhanath, Hong Kong & Esby: Rangjung Yeshe, 1999), Chapter 1 'The Basis: Buddha Nature'.