Using a reduced model focusing on the in-plane dependence of plane Couette flow, it is shown that the turbulent → laminar relaxation process can be understood as a nucleation problem similar to that occurring at a thermodynamic first-order phase transition. The approach, apt to deal with the large extension of the system considered, challenges the current interpretation in terms of chaotic transients typical of temporal chaos. The study of the distribution of the sizes of laminar domains embedded in turbulent flow proves that an abrupt transition from sustained spatiotemporal chaos to laminar flow can take place at some given value of the Reynolds number R low , whether or not the local chaos lifetime, as envisioned within low-dimensional dynamical systems theory, diverges at finite R beyond R low . PACS: 47.20.Ft, 47.27.Cn, 05.45.Jn The transition to turbulence in flows lacking linear instability modes, such as Poiseuille pipe flow driven by a pressure gradient along a circular tube (Ppf) and plane Couette flow driven by two plates moving parallel to each other in opposite directions (pCf), is particularly delicate to understand owing to its abrupt character, without the usual cascade seen in the globally super-critical case, as for e.g. convection. A recent general presentation of the issues is given in [1]. These globally sub-critical flows become turbulent through the nucleation and growth or decay of turbulent domains called puffs (Ppf) or spots (pCf), see, e.g., [2] and [3] for Ppf and pCf, respectively. Most of the work on the transition problem has dealt with special nonlinear solutions (exact coherent structures [4, b]) to the Navier-Stokes equations and their dynamics in the phase space spanned by them. Such solutions, obtained within the so-called Minimal Flow Unit (MFU) assumption [5], have been found at moderate values of the Reynolds number R in Ppf, [6], pCf [4,[7][8][9], and also in plane Poiseuille flow [8,10]. (In pCf, R is defined as Uh/ν where U is the speed of the plates driving the flow, 2h the gap petween the plates, and ν the kinematic viscosity of the fluid.) These solutions are all unstable and, together with their stable and unstable manifolds, they form the skeleton of the turbulent flow at a local scale. The reason why turbulence can be sustained only at much higher values of R (about a factor of two to three higher) is however not clear [2].