In plain words
Thoughts come and go. So do moods, sensations, memories and even the feeling of being a particular person. Many contemplative traditions point to something that notices all of this without being any of it: a quiet awareness in which experiences appear and pass. That noticing presence is often called the witness.
The idea is easy to state and hard to hold. A thought can be watched, but the one watching cannot be found as another thought. Some traditions treat the witness as a person's true nature. Others treat it as a trained quality of attention and deny that any lasting self stands behind it. Several of the sources below also move past the picture of a detached observer, toward a seeing in which watcher and watched are no longer two.
Across the traditions
Advaita Vedanta (Hinduism)
Its word: sākṣin (often written sakshi), "the witness."
In a line: the pure consciousness that sees body, mind and world and is touched by none of them.
Advaita Vedanta, the "not two" school taught by Adi Shankara, holds that atman, the innermost self, is one with Brahman, the ultimate reality. A verse attributed to Shankara describes the pure consciousness that shines through waking, dream and deep sleep as the witness of the whole universe. The Ashtavakra Gita, an anonymous Sanskrit text spoken in the voice of the sage Ashtavakra, puts it most bluntly: "You are the one witness of everything and are always completely free. The cause of your bondage is that you see the witness as something other than this." (1.7, trans. John Richards)
Theravada Buddhism
Its word: vipassana, "insight."
In a line: meditation that watches thoughts and feelings rise and pass away.
Dipa Ma, a Bengali teacher in the Burmese vipassana lineage, reduced the practice to a single instruction carried into cooking, ironing and conversation: "Whatever you are doing, be aware of it." (as recorded by Amy Schmidt) Theravada does not treat this awareness as a permanent self. Its teaching of anattā, not-self, holds that no lasting watcher stands behind the watching.
Zen Buddhism
Its word: wunian, "nonthought."
In a line: seeing everything clearly while the mind sticks to nothing.
Huineng, the Sixth Patriarch of Chan, did not ask students to stop thinking. In the Platform Sutra he defines the term: "What is nonthought? If in seeing all the dharmas, the mind is not defiled or attached, this is nonthought." (trans. John R. McRae) The emphasis falls less on a separate watcher than on seeing that does not cling: thought goes on, and nothing gets stuck.
Sufism (Islam)
Its word: mushāhada, "witnessing."
In a line: the heart's direct seeing of God's oneness.
In Sufism, the mystical side of Islam, the witness is not a neutral onlooker but a heart that sees divine unity directly. Abū al-Ḥasan al-Shādhilī, the thirteenth-century founder of the Shadhiliyya order, left no books, and his teaching survives through his spiritual grandson Ibn ʿAṭāʾ Allāh. Asked about the mysteries of God's oneness, he is reported to have answered with a rule for speech and inner sight that pairs farq, distinction, with jamʿ, union: in speech the creature and God are kept distinct, as the creed requires, while inwardly the heart witnesses union.
