Meaning 'that which is not perceived by the 4 senses'. Sensed by feeling, knowing or by intuition.
It's a funny answer and it might be a philosophical one but I hope it will be helpful. I'm coming from 25 years of deep study in this subject.
I posit that there are four main different parts of the self. I call these:
The inner masculine - doing, thinking acting
The inner feminine - feeling, relating, wishing, dreaming
The inner child - creating, playing
The higher self - being, spiritual, meditating, yoga
Each if these parts is like 4 separate people, inside each person. Each has its own completely separate belief system with its own wishes, desires, perceptions, issues, neuroses, goals, and so on.
This internal separation is functional - these four parts are supposed to work together as a team - however in most people, because of ancient trauma in the human race, held in collective consciousness, they don't really work together at all.
Many people are aware only of the 'inner masculine' (Freud's 'ego') and value only that part of the self. This is part of the dysfunction that has caused humanity so much grief.
You may conceive of the other three parts of the self - the child, the feminine, and the higher self - as being 'metaphysical and non-real'.
They are - and they are also the poor relations within most people - the integration and connection with these parts would bring the wholeness that most people are seeking in their lives.
So in answer to your question I suggest 'the un-seen'.
And I offer my own book, which explores the 4 parts of self I have discovered, as a reference. It is a 25 year deep study into healing through integrating these 4 parts, written in a simple way.
I love that you have asked this question.