Alterable space is a particular phenomenon and theory in the Star Ocean series.
When matter exists in three-dimensional space, space warps in direct proportion to the amount of energy in that matter, and that matter begins moving in a direction described by a curved, or geodesic line. This phenomenon appears to observers as gravity, and acts as if a force were being exerted in the surrounding area. "Alterable space" (or "altspace" for short) is a term referring to this type of three- dimensional universe.
People tend to think that the mass in matter warps space and creates gravity, but in reality, it is the energy in the matter that actually warps space, thereby giving the appearance of what we refer to as "mass".
The theory of alterable space was a historical scientific breakthrough, and led to a major revolution that resulted in the recent gravity-based technologies such as Gravitic warp, all of which depend upon total control over altspace.