An a-type space is a space with a single link. It is the dead-end space through which no movement is possible to other spaces.
Cross-References
Recommended Reading
Space Syntax Glossary
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;
Hillier, B. (1996, 2007), Space is the Machine: A Configurational Theory of Architecture. Space Syntax: London, UK. p1, p23