Hyperspace is the most physical and empiric medium through which ships travel faster than light (ftl), making interstellar exploration and civilisation possible. It is intermediate between realspace and paraspace, and accessed through hyperdimensional rotation, which means a ship rotates through an additional dimension or dimensions of space and time and finds itself in an ontologically distinct reality frame.
It is also a generic term for higher dimensional (hyperspace and hypertime) realities superimposed on the mundane dimensions of spacetime, but invisible and intangible to us.
It differs from realspace primarily in having one or more dimensions of ontological zone along which movement is possible. Also, in addition to the usual three dimensions of space and one of time, there may also be one or more additional dimensions of space (hyperspace) and/or time (hypertime). as a sort of counter physical space.