the middle: the central part of something, or the space it encloses
At its core this word names a middle, the central part of a thing or the space shut in by it. In practice the Hebrew almost always sets a small word in front of it to say where something stands in relation to that middle, and the senses below are points of view on that one idea.
With "in" it means inside or within, the middle of a solid object, a body of water, a city, or a fire. With the same "in" but a group of people it means among or in the midst of them, the way the Bible speaks of God dwelling among Israel. With "out of" it means from within or from among, the movement of coming out of that middle, like the voice that called from inside the burning bush.
Because it points to a location rather than naming a kind of thing, English translators reach for many small words for it (midst, middle, among, within, inside, from among), but the underlying picture is always the same center or enclosed space.