Design space is a concept from engineering design: the collection of all possible designs for a thing, considered as a high-dimensional space parametrized by design decisions. This concept is intimately related to design Optimization, since a space of possible designs must be identified before an optimal design can be selected. The use of a spatial metaphor allows evolution to be visualized as a trajectory in design space. See Evolution as Algorithm and Darwin's Dangerous Idea.
Design space also implies the idea of a Tradeoff because a design with one or more desirable properties may be feasible (in the design space), but any single design cannot be optimized for all qualities simultaneously.