A d-type space is one with more than two links and which forms part of systems which contain neither a- nor b-type spaces, and which therefore must contain at least two rings which have at least one space in common. Such a space must lie on more than one ring.
d-space or d type space
Hillier, B. (1996, 2007), Space is the Machine: A Configurational Theory of Architecture. Space Syntax: London, UK. pp.250-1