Many healing modalities today talk about the body - how emotions and memories can be connected to the body, how we sense many things through the body instead of the mind, how groundedness and intuition have more to do with body than with thought and theory. The concept of being "in your body" or "out of your body" is very common, and in my mind very useful.
However, having started life as a mathematician, I am uncomfortable with unclear definitions, and have had to think a lot about what people really meant when they used the word "body" in this new sense. The following is how I have finally come to view it:
First, we are not talking about "the body" in the normal sense - fingers, toes, hair, gall bladder, red blood cells, white blood cells. We are talking about sensations in the body. Second, we are not talking about normal sensations - itches, tickles, pressure, heat, cold. We are talking about certain sensations, usually vague, usually emanating from some location internal to the head or trunk that is not accessible through touch. The concept of location also cannot be taken literally - there is no way of knowing of those sensations are being produced by the body or by the brain. All we know is that it "feels" like it resides in the body.
I suspect that we are talking about what Gendlin brilliantly calls "felt sense":
Much of what a person knows has never been consciously thought or verbalized. Felt sense is the name Gendlin gave to the unclear, pre-verbal sense of 'something', as that something is experienced in the body. It is not the same as an emotion. This bodily felt 'something' may be an awareness of a situation or an old hurt, or of something that is 'coming' — perhaps an idea, or the next line of a poem, or the right line to draw next in completing a drawing. Crucial to the concept, as defined by Gendlin, is that it is unclear and vague; and it is always more than any attempt to express it verbally.
So we can know something, and know it very deeply, and yet have no words to adequately describe the whole of what we are feeling. Not that we haven't found the right words yet; rather, the experience in its fullness is bigger and more complex than what logical concepts are capable of construing.
The very experience of being alive, of having a body of constant activity - blood pulsing, heart pumping, food digesting, lungs filling with air, cells splitting, must create an overall body sensation we have known since before birth, and which we are so used to that we rarely even notice it. Yet when we are feeling "alive", when we lift our arms and breath deep, and feel that life is joyous, we can sense the feeling of life within us. This is being in our body - to have a connection with that familiar sensation that we have known since the beginning.