Definition - What does Witness mean?
A witness is a person who has or is thought to have seen and can provide significant information about an action or occurrence.
In yoga, the term refers to transcendental consciousness - observing the mind from deep inside - and is usually used in the terms "inner witness" and "witness consciousness."
Yogapedia explains Witness
To be the witness of one's own mind and thoughts is one of the most significant parts of a yoga practice. It is most important during meditation, when being one's witness means being able to peacefully observe the flow of the mind with no distraction or disturbance. The process of witnessing can be simplified as a combination of observing and nonattachment. It is the observance of the mind and circumstances, without being attached, affected or involved. The witness does not interfere and is not afflicted or disturbed by what he/she observes.
The Sanskrit term for witness is sakshi. It is a combination of the words, sa, meaning "with," and aksha, meaning "senses."