A single isovist is the volume of space visible from a given point in space, together with a specification of the location of that point. Isovists are naturally three-dimensional, but they may also be studied in two dimensions: either in horizontal section ("plan") or in other vertical sections through the three-dimensional isovist. Every point in physical space has an isovist associated with it.
The boundary-shape of an isovist may or may not vary with location in, say, a room. If the room is convex (for example, a rectangle or circle), then the boundary-shape of every isovist in that room is the same; and so is its volume (or area, if we are thinking in plan). But the location of the viewpoint relative to the boundary would or could be different. However, if the room were non-convex (for example, an L-shaped or partitioned room), then there would be many isovists whose volume (area) would be less than that of the whole room, and perhaps some that were; and many would have different, perhaps unique shapes: large and small, narrow and wide, centric and eccentric, whole and shredded.
One can also think of the isovist as the volume of space illuminated by a point source of light.
It is used in the field of architecture for analysis of buildings and urban areas, typically as one of a series of methods used in space syntax.