Chreod

A chreod (from Greek khreia, "necessity" + hodos, "path") is a stable developmental trajectory — a constrained channel through which a developing system proceeds toward a particular endpoint. The term was introduced by Conrad Waddington as part of his epigenetic landscape metaphor, where chreodes are the valleys that guide a cell through differentiation despite perturbation.

In modern dynamical systems terms, a chreod is a developmental attractor: a region of state space toward which the system converges from diverse initial conditions. The depth of a chreod — its resistance to perturbation — is what Waddington called canalization. The chreod concept raises an unresolved question: is development genuinely convergent (many paths, one end) or merely robust (one path, well-guarded)? The distinction matters because convergence hides more genetic variation from selection than robustness does.