In the Zen tradition of Non-Duality it is stated that there is One Mind. But what is the mind? It can't really be defined. No nouns can really be defined. The reason being that Infinity is the reality behind the seeming limits of the world.
So defining the mind is impossible. People generally have a vague assumption of what the mind is, and this assumption is generally accepted and shared by people to different degrees. That means that people have an idea of what the mind is, and some people have similar ideas about it, but there is still a vagueness. There is the materialist view that matter is the main stuff of the world, and that the human form evolved with a brain that creates the mind. But the Non-Duality understanding is very different to the materialist.
Truly all we can know about the world, the human form and the brain is known in the mind. We can't know anything without the mind - that aware, knowing, perceiving ability. The mind can be considered as a focusing of sensation and perception. We could say the mind is a qualityless, spaceless space in which thoughts, sensations and perceptions appear. Thinking appears in the mind. Hearing, seeing and all other sensing appears in the mind. The whole world appears in the mind. What seems to be outside and inside all appear in the mind.
The human mind generally has a sense of limitation, being a centre point at the heart of perception. We can say that the mind is that seemingly limited field in which a viewpoint of the world is perceived. But the perceived world is not separate from the mind. Neither is perceiving. It can be recognised that the mind is perceiving an appearance of centralisation. It doesn't mean that this centralisation is true or real. That's just how it appears. The belief of this centralisation develops the sense of a limited, separate self - me and the world. This belief also appears in the mind.
We can go deeper. We can perceive this sense of identity. We can perceive thinking. We can perceive the body's sensations and its perceptions, which include the appearance of the world. We don't really perceive limits to the mind. The edge of the mind can't be found. Indeed, the mind only imagines its limitations.
In Non-Duality understanding there is only Awareness or Consciousness. In Zen this is the One Mind. The seemingly limited mind is perceived in Awareness. There is One Awareness. We could say it is like a central pool from which all mind streams flow. But truly there is no centre. There is only One Mind, giving the appearance of centre and outer. There is no inside and outside. These opposites seem to appear in the mind, but there is no true limiting that divides the mind. The 'outer' world can never be experienced outside of the mind, because there is no outside of the mind.
All that is is One Mind. Even the term 'Mind' isn't it, but it's a way of trying to describe it. Even 'One' isn't accurate. Truly there is no number. There is just What Is.