From an HCI perspective , a design space is a tool that signals the different possibilities for designing a certain type of artefacts, supporting and augmenting the design practice. Designers create a design space as a reflection on the properties of, and the design choices made for, existing artifacts used for a similar purpose. A design space supports the creation of new artefacts along the lines of a set of dimensions for which it proposes multiple values. It also learns from each new design experience by enhancing the existing dimensions with new design possibilities. In this way, the design dimensions respond to the realities, possibilities and concerns of the design discipline.
Just as it’s used for human-computer interaction (HCI), a design space can be built for other disciplines/areas. Today I found a great example of a design space and analysis for the characters of a comic strip I follow, Niels, according to some of their most salient characteristics: criminal tendency, sexuality, fighting skills, and transparency. See below.
In a few lines, if you want to define your design space you have to identify the relevant characteristics of what you’re designing, define them, define their possible values, match your design objects within such categories, and then analyse the whole set (don’t miss the analysis graph at the bottom of Niels’ cartoon). Good examples in my own work are the design space analysis for InterruptMe (UIST 2011 ) and The Rabbit (Ubicomp 2011 ).
 – Allan Maclean, Richard Young, Victoria Bellotti, and Thomas Moran. Design space analysis: Bridging from theory to practice via design rationale. Esprit, pages 720–730, 1991. URL: citeseer.ifi.unizh.ch/article/maclean91design.html.
 – J. D. Hincapié-Ramos, S. Voida, and G. Mark, “A Design Space Analysis of Interrupter–Interruptee Trade-Offs in Availability-Sharing Systems,” in UIST 2011, Oct 16-19, 2011, Santa Barbara, CA, USA. To Appear.
 – J. D. Hincapié-Ramos, A. Tabard, and J.E. Bardram, “Mediated Tabletop Interaction in the Biology Lab — Exploring the Design Space of The Rabbit,” in Ubicomp 2011, Sep 17-21, 2011, Beijing, China. To Appear.