In plain words
Emptiness is the teaching that nothing exists on its own, as a fixed and separate thing. A cup, a mountain or a person is real as it appears, yet each depends on causes, parts and conditions, and none has a core that stands apart from everything else. Mystics use the word for what is left when the mind stops grasping at that imagined core: an open, unobstructed space in which things go on appearing.
Emptiness is not nihilism. It does not say that nothing exists, that nothing matters or that life is meaningless. The Buddhist philosopher Nagarjuna, writing around the second century, taught that emptiness and dependent arising are the same fact seen from two sides, and he compared emptiness grasped the wrong way to a snake seized by the wrong end. In devotional traditions the emptying is personal. Christian writers speak of kenosis, "self-emptying," from Paul's letter to the Philippians (2:7), and Sufis speak of the self passing away so that God alone remains.
Across the traditions
Buddhism: śūnyatā
Śūnyatā (Sanskrit, "emptiness") means that all things are empty of "own-being," any nature of their own apart from causes and conditions. It grows from the teaching of not-self given by Siddhartha Gautama (the Buddha) and stands at the center of the Mahayana Perfection of Wisdom sutras. The shortest of them, the Heart Sutra, puts it in one line: "form is emptiness and the very emptiness is form" (trans. Edward Conze).
Zen: kong
Chan, known in Japan as Zen, kept the teaching and turned it toward the mind itself: kong (Japanese kū) is the uncluttered nature already present before any effort to purify it. When the Emperor Wu asked for the first principle of the sacred doctrine, Bodhidharma is said to have answered, "It's just empty; there's nothing sacred" (as rendered by Alan Watts). The Platform Sutra gives the verse of Huineng: "Fundamentally there is not a single thing—where could any dust be attracted?" (trans. John R. McRae).
Taoism: xu and wu
In the Tao Te Ching, xu (emptiness) and wu (non-being) are what make things useful and keep them alive. The book credited to Laozi says, "The Tao is (like) the emptiness of a vessel; and in our employment of it we must be on our guard against all fulness" (ch. 4, trans. James Legge, 1891). It praises the "valley spirit" that "dies not" (ch. 6), and the wheel, pot and room whose use depends on their "empty space" (ch. 11).
Sufism: fana
Fana (Arabic, "passing away") is the emptying of the separate self in God. Bayazid Bastami, a ninth-century Persian Sufi, was among the first to make it the center of the path, and among the first to describe the mystic's experience as a mi'raj, an ascent like the Prophet's night journey. Early accounts of that ascent describe him passing beyond every quality and relation until only the Real, God, remained. The Sufi tradition reads such accounts as the report of a self that has vanished, not as a boast.
Virashaiva Hinduism: shunya
In the twelfth-century Kannada poems of the Virashaiva movement, shunya (the void, zero) and bayalu (open space) name the final goal, beyond every image of God. Allama Prabhu, seated by tradition on the Shunya Simhasana, the "throne of the void," turned the idea even against the sacred objects his own movement prized. In one of his best-known poems he goes looking for God's light and ends by setting aside every comparison, since nothing can stand for it.
