Cumulative isovist is the mean fraction of building area that could have been viewed by an agent with 360 degree vision during its visit. This variable gives an idea of how much an agent optimises their explorative ability.
Turner, A. and Penn, A. (2002) Encoding Natural Movement as an Agent-based System: an Investigation into Human Pedestrian Behaviour in the Built Environment. Environment and Planning B, 29(4), 473-490. pp.483-484.