Chreod, is a portmanteau term coined by 20th century biologist Conrad Hal Waddington that combines the Greek word for "determined" or "necessary" and the word for "pathway." The term was used along with homeorhesis, which describes a system that returns to a steady trajectory in contrast to homeostasis which describes a system which returns to a steady state.

Architectural Theorist Sanford Kwinter described the concept of the chreod as "the most important concept of the 20th century."[1]

The word "chreod" also closely describes paths of decision within what Christopher Alexander has called, configuration space, his term for what he notes that Stuart Kaufmann calls "ﬁtness landscape."[2] Configuration space is a conceptual landscape which contains all possible outcomes and points of decision within the design of a structure or the growth of an organism or system. Because so very few of the possible outcomes will be good ones, paths to the infinitesimally rare possible and good solutions must be followed to get good results. These paths or, "chreods" appear to be followed automatically in the biological world. (see the cited Kwinter excerpt) By Alexander's theory, because conscious human design decisions do not need to follow these chreods, conscious human design can lead to mixed results. Therefore, he proposes that discovering ways to allow architecture to follow these paths is the best way to get good results in the built environment. Alexander sees his theories of "The Fundamental Process," "structure preserving transformations" and "15 fundamental properties" which he outlines in his four volume work, The Nature of Order as instrumentally shaping paths through configuration space.