Graph is a collection of points and lines connecting some subset of them. In space syntax, the points of a graph denote spaces (e.g. streets, rooms or corridors), and the lines represent intersection between pairs of the spaces.
Hillier, B., Hanson, J., and Graham, H. (1987), Ideas are in things: an application of the space syntax method to discovering house genotypes. Environment and Planning B: Planning and Design, v14, 363-385. pp. 363-4.